Closes #5558

Closes #5267

Helped-by: oni-link <knil.ino@gmail.com>

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).

move `call_shell` to misc1.c
Move some fns to state.c
Move some fns to option.c
Move some fns to memline.c
Move `vim_chdir*` fns to file_search.c
Move some fns to new module, bytes.c
Move some fns to fileio.c

- rename to shell_xescape_xquote
- move to os/shell.c
- disallow NULL argument
- eliminate casts, nesting
- test: empty shellxquote/shellxescape

Fixes #2773

As discussed on #5243 and #5283.

Helped-by: John Szakmeister <john@szakmeister.net>
Helped-by: Justin M. Keyes <justinkz@gmail.com>

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.