fix(tui): call tcdrain() on stdout and stderr on exit (#38154)

Problem: On FreeBSD, output written to TTY may be lost on exit. Example test failure:

FAILED test/functional/terminal/tui_spec.lua @ 2521: TUI no assert failure on deadly signal #21896 test/functional/terminal/tui_spec.lua:2523: Row 1 did not match. Expected: |Nvim: Caught deadly signal ‘SIGTERM’ | | | |[Process exited 1]^ | | | |* | | | |{5:– TERMINAL –} | Actual: |* | |[Process exited 1]{100:^ }| |{100:~ }| |{100:~ }| |{3:[No Name] }| | | |{5:– TERMINAL –} | To print the expect() call that would assert the current screen state, use screen:snapshot_util(). In case of non-deterministic failures, use screen:redraw_debug() to show all intermediate screen states. Snapshot: screen:expect([[ | [Process exited 1]{100:^ }| {100:~ }|*2 {3:[No Name] }| | {5:– TERMINAL –} | ]]) stack traceback: test/functional/ui/screen.lua:909: in function ‘_wait’ test/functional/ui/screen.lua:537: in function ‘expect’ test/functional/terminal/tui_spec.lua:2523: in function <test/functional/terminal/tui_spec.lua:2521>

Solution: Call tcdrain() on stdout and stderr on exit.

This problem is only observed on FreeBSD, but it probably doesn’t hurt to do this on all platforms with termios.h. In fact using tcdrain() on PTY slave is no-op on Linux according to Linux kernel source code.

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├─ cmake/           CMake utils
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