Make sure terminal settings are restored before exiting

Make sure terminal settings are restored before exiting

When exiting GDB -- whether it's via the "quit" command, via a SIGTERM,
or otherwise -- we should leave the terminal in the state we acquired
it.  To that end, we have to undo any modifications that may have been
made by the TUI (ncurses) or by the CLI (readline).

Tested on x86_64 Debian Stretch.

gdb/ChangeLog:

	* top.c: Include "tui/tui.h".
	(undo_terminal_modifications_before_exit): New static function.
	(quit_force): Use it.

gdb/testsuite/ChangeLog:

	* gdb.base/batch-preserve-term-settings.exp
	(test_terminal_settings_preserved_after_cli_exit): New test.