Show a proper confirmation dialog when trying to unload a terminal buffer while
the confirm option is set or when :confirm is used.

Fixes https://github.com/neovim/neovim/issues/4651

Fix redrawing issues with narrow screen and remove extra wait in resize tests

Before this, bottom of screen got messed up when modemsg (+ one extra
space to not force terminal scroll) didn't fit on one line.

Saves ~10 seconds in UI tests

Now that uv.h is directly being included, pre-processing of
test/includes/pre/uv.h fails on Linux with

    In file included from «SRCDIR»/neovim/test/includes/pre/uv.h:1:
    In file included from /usr/include/uv.h:62:
    /usr/include/uv/unix.h:72:11: fatal error: 'uv/pthread-barrier.h' file not found
    # include "uv/pthread-barrier.h"
              ^~~~~~~~~~~~~~~~~~~~~~
    1 error generated.

This happens because we're missing -D_GNU_SOURCE (part of ${gen_cdefs}),
which makes the pthread_barrier_* functionality visible.

libuv users are only supposed to directly include uv.h.  In v1.21.0, all
the uv-*.h headers were renamed to uv/*.h, which caused the unit tests
to fail with

    [123/125] Generating post/uv-errno.h
    FAILED: test/includes/post/uv-errno.h
    cd «SRCDIR»/src/neovim/build/test/includes && /usr/bin/clang -std=c99 -E -P «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h -I/usr/include -I/usr/include -o «SRCDIR»/neovim/build/test/includes/post/uv-errno.h
    «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h:1:10: error: 'uv-errno.h' file not found with <angled> include; use "quotes" instead
    #include <uv-errno.h>
             ^~~~~~~~~~~~
             "uv-errno.h"

The intention of the file is to extend libuv's error constants with more
values used by the unit tests.  This can just as easily be achieved
without poking into pseudo-private header files.

closes #8466
closes #8664
Regression by 0d7daaad98d5.

- Fix length comparison.
- Fix loop(s) which iterated over all fields of array `pcc` even if it
  was not filled up (try unicode 0x9f as statusline character).

Note about the tests:
- To input unicode with more than two hex digits you can use <C-v>U...:
  a + U+fe20: a︠
  a + U+fe20 + U+fe21: a︠︡

Improve coverage of `build_stl_str_hl`.
Minor removal of dead code in the tested function.