Test that terminal settings are restored after quitting via SIGTERM

Test that terminal settings are restored after quitting via SIGTERM

Tested on x86_64 Debian Stretch, native, gdbserver and
extended-gdbserver.  Also tested that the various error paths, like if
$PPID is empty or if SIGTERM did not not kill GDB, function correctly.

gdb/testsuite/ChangeLog:

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