ui_bridge.c is useful for libnvim consumers, not just the built-in TUI.

This reverts commit beb2e4f095583af09ebe9c66e3bf453b61511f23.

unibi_format() calls out() multiple times for a given format string.
When data->buf fills up during this process, flush_buf() gets called,
which possibly calls unibi_out() again to toggle the cursor visibility.
However, if we were halfway through outputting an escape sequence, doing
this will clobber it, resulting in junk being displayed.

Fix this by not toggling the cursor visibility when draining a full
buffer in out().

References #4867

For users who use a "bar" shape in the shell, it's annoying that Nvim starts
with that same cursor shape, despite starting in normal-mode. So default to the
normal-mode "block" shape instead. (Note: technically it's possible some user
may set 'insertmode', and then the opposite problem occurs. But 'insertmode' is
a silly option that shouldn't exist, and any user that uses it probably isn't
fiddling with their cursor shape anyways.)

Also rename the unibilium extensions:
    enter_insert_mode   => set_cursor_shape_bar
    enter_replace_mode  => set_cursor_shape_underline
    exit_insert_mode    => set_cursor_shape_block
to say explicitly what they do in the context of a terminal; it's irrelevant in
this context what purpose they serve in Nvim.

`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

Closes #4163

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