Restore terminal state in mi_thread_exit (PR gdb/17627)

Restore terminal state in mi_thread_exit (PR gdb/17627)

When a thread exits, the terminal is left in mode "terminal_is_ours"
while the target executes.  This patch fixes that.

We need to manually restore the terminal setting in this particular
observer.  In the case of the other MI observers that call
target_terminal_ours, gdb will end up resuming the inferior later in the
execution and call target_terminal_inferior.  In the case of the thread
exit event, we still need to call target_terminal_ours to be able to
print something, but there is nothing that gdb will need to resume after
that. We therefore need to call target_terminal_inferior ourselves.

gdb/ChangeLog:

	PR gdb/17627
	* target.c (cleanup_restore_target_terminal): New function.
	(make_cleanup_restore_target_terminal): New function.
	* target.h (make_cleanup_restore_target_terminal): New
	declaration.
	* mi/mi-interp.c (mi_thread_exit): Use the new cleanup.

Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>