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

Fixes #4151

libvterm uses an "output buffer" for terminal reporting
(e.g. \e[6n to report cursor position)
Flush it in on_channel_output() not just terminal_send_key()

See also this line from pangoterm:
https://bazaar.launchpad.net/~leonerd/pangoterm/trunk/view/607/pangoterm.c#L2159

Sometimes an extra FocusGained/FocusLost event might trigger. This
doesn't matter, we just want to test that the events were received in
cmdline-mode.

closes #8096

closes #2454
closes #8213
ref #7972

closes #7698

Wrapping a command in double-quotes allows cmd.exe to safely dequote the
entire command as if the user entered the entire command in an
interactive prompt. This reduces the need to escape nested and uneven
double quotes.

The `/s` flag of cmd.exe makes the behaviour more reliable:

    :set shellcmdflag=/s\ /c

Before this patch, cmd.exe cannot use cygwin echo.exe (as opposed to
cmd.exe `echo` builtin) even if it is wrapped in double quotes.

Example:
:: internal echo
> cmd /s /c " echo foo\:bar" "
foo\:bar"

:: cygwin echo.exe
> cmd /s /c " "echo" foo\:bar" "
foo:bar

Update tests.

Make HlAttr contain highlighting state for both color modes (cterm and rgb).
This allows us to implement termguicolors completely in the TUI.

Simplify some logic duplicated between ui.c and screen.c. Also avoid
some superfluous highlighting reset events.

The old behavior is probably not justified, for the usual reason:
terminal buffers may have interactive processes, so cursor placement is
arbitrary, therefore tracking it in the jumplist is useless (or worse).

N.B.: per the docstring for `checkpcmark()` it looks like we were
calling `checkpcmark()` and `setpcmark()` in the wrong order.

closes #3723