Closes #3529
Closes #5241

In Vim,
    :echo system('cat - &', 'foo')
works because for both system() and :! Vim writes input to a temp file and uses
shell syntax to redirect the file to the backgrounded `cat` (get_cmd_output()
.. make_filter_cmd()).

In Nvim,
    :echo system('cat - &', 'foo')
fails because we write the input directly via pipes (shell.c:do_os_system()),
but (per POSIX[1]) backgrounded process input stream is redirected from
/dev/null (unless overridden by shell redirection; supported only by some shells
[2]), so our writes are ignored, the process exits quickly, and if we are
writing data larger than the buffer size we'll see EPIPE.

This still works:
    :%w !tee > foo1358.txt &
but this does not:
    :%w !tee foo1358.txt &
though it *should* (why doesn't it?) because we still do the temp file dance
in do_bang() .. do_filter().

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_03_02
[2] http://unix.stackexchange.com/a/71218

`lib/queue.h` implements a basic queue. `event/queue.c` implements
a specialized data structure on top of lib/queue.h; it is not a "normal"
queue.

Rename the specialized multi-level queue implemented in event/queue.c to
"multiqueue", to avoid confusion when reading the code.

Before this change one can eventually notice that "macros (uppercase
symbols) are for the normal queue, lowercase operations are for the
multi-level queue", but that is unnecessary friction for new developers
(or existing developers just visiting this part of the codebase).

Because terminfo_stop() already ran, there is not much reason to wait
for the loop to teardown.

Helped-by: Björn Linse <bjorn.linse@gmail.com>
Helped-by: oni-link <knil.ino@gmail.com>

Closes #4778
References #3541
---
Bug report:

> After pressing `ZZ` I can find two threads freezing, occupying 100% CPU:
    |-systemd-+
              |-nvim,11567 /home/lz/code/1.rs +set title
              |   `-{nvim},11574

> 11567 has two threads:
    (gdb) info threads
      Id   Target Id         Frame
    * 1    Thread 0x7f7622907780 (LWP 11567) "nvim" 0x00007f76222e66bd in pthread_join () from /usr/lib/libpthread.so.0
      2    Thread 0x7f761f5ff700 (LWP 11574) "nvim" 0x00007ffcec9e9c59 in clock_gettime ()

    (gdb) thread apply all  bt

    Thread 2 (Thread 0x7f761f5ff700 (LWP 11574)):
    #0  0x00007ffcec9e9c59 in clock_gettime ()
    #1  0x00007f76210b9356 in clock_gettime () from /usr/lib/libc.so.6
    #2  0x00007f7622513e3c in ?? () from /usr/lib/libuv.so.1
    #3  0x00007f7622505e94 in uv_run () from /usr/lib/libuv.so.1
    #4  0x00000000004822ed in loop_close (loop=loop@entry=0x7f761f5fe870) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/event/loop.c:87
    #5  0x00000000005a7ec0 in tui_main (bridge=0x7f761f6ac000, ui=0x7f761f69ace0) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/tui/tui.c:234
    #6  0x00000000005a9b47 in ui_thread_run (data=<optimized out>) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/ui_bridge.c:87
    #7  0x00007f7622510d07 in ?? () from /usr/lib/libuv.so.1
    #8  0x00007f76222e5474 in start_thread () from /usr/lib/libpthread.so.0
    #9  0x00007f76210ac69d in clone () from /usr/lib/libc.so.6

    Thread 1 (Thread 0x7f7622907780 (LWP 11567)):
    #0  0x00007f76222e66bd in pthread_join () from /usr/lib/libpthread.so.0
    #1  0x00007f7622510dae in uv_thread_join () from /usr/lib/libuv.so.1
    #2  0x00000000005aac9e in ui_bridge_stop (b=0x7f761f6ac000) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/ui_bridge.c:104
    #3  0x00000000005a90e4 in ui_builtin_stop () at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/ui.c:91
    #4  0x000000000052be09 in mch_exit (r=r@entry=1) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os_unix.c:143
    #5  0x00000000004db1cc in getout (exitval=exitval@entry=1) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/main.c:623
    #6  0x00000000004fa43c in preserve_exit () at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/misc1.c:2652
    #7  0x000000000052b77a in deadly_signal (signum=1) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os/signal.c:120
    #8  0x000000000052b7cf in on_signal (handle=<optimized out>, signum=<optimized out>, data=<optimized out>) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os/signal.c:145
    #9  0x0000000000484178 in signal_event (argv=<optimized out>) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/event/signal.c:44
    #10 0x0000000000483b7f in queue_process_events (queue=0x7f7620417360) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/event/queue.c:142
    #11 0x0000000000482208 in loop_poll_events (loop=0x84dec0 <loop>, ms=ms@entry=4000) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/event/loop.c:56
    #12 0x000000000052a364 in input_poll (ms=ms@entry=4000) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os/input.c:325
    #13 0x000000000052a3e2 in inbuf_poll (ms=4000) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os/input.c:347
    #14 0x000000000052a839 in os_inchar (buf=buf@entry=0x0, maxlen=maxlen@entry=0, ms=ms@entry=-1, tb_change_cnt=tb_change_cnt@entry=0) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/os/input.c:107
    #15 0x0000000000592eeb in state_enter (s=s@entry=0x7ffcec9d3560) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/state.c:46
    #16 0x0000000000508533 in normal_enter (cmdwin=cmdwin@entry=false, noexmode=noexmode@entry=false) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/normal.c:464
    #17 0x00000000004dc17e in main (argc=<optimized out>, argv=<optimized out>) at /tmp/yaourt-tmp-lz/aur-neovim-git/src/neovim-git/src/nvim/main.c:538

Inherited signal mask may block SIGCHLD, which causes libuv to hang at
epoll_wait.

Closes #5230

Helped-by: Nicolas Hillegeer <nicolas@hillegeer.com>
Helped-by: John Szakmeister <john@szakmeister.net>

Note: the #pragma gymnastics are a workaround for broken system headers on
macOS.

  signal.h:
      int  sigaddset(sigset_t *, int);
      #define     sigaddset(set, signo)   (*(set) |= __sigbits(signo), 0)
  sys/_types/_sigset.h:
      typedef __darwin_sigset_t            sigset_t;
  sys/_types.h:
      typedef __uint32_t   __darwin_sigset_t;      /* [???] signal set */

sigset_t is defined as unsigned int, but the sigaddset() ORs it with an int,
mixing the types.  So GCC generates a sign-conversion warning:

  sig.c:9:13: warning: implicit conversion changes signedness: 'int' to 'unsigned int' [-Wsign-conversion]
    (*(&s) |= __sigbits((sigset_t) 20), 0);
           ~~ ^~~~~~~~~~~~~~~~~~~~~~~~
  1 warning generated.

System headers are normally ignored when the compiler generates warnings:
https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html

  > GCC gives code found in system headers special treatment. All warnings,
  > other than those generated by ‘#warning’ (see Diagnostics), are suppressed
  > while GCC is processing a system header. Macros defined in a system header
  > are immune to a few warnings wherever they are expanded. This immunity is
  > granted on an ad-hoc basis, when we find that a warning generates lots of
  > false positives because of code in macros defined in system headers.

Instead of the #pragma workaround, we could cast the sigset_t pointer:

    # if defined(__APPLE__)
      sigaddset((int *)&mask, SIGCHLD);
    # else
      sigaddset(&mask, SIGCHLD);
    # endif

but that could break if the headers are later fixed.

Because the old name did not indicate that the function
would return true on directories as well.

Fix for missing output (#4569, ...)