Make target_terminal_inferior/ours almost nops on non-main UIs

Make target_terminal_inferior/ours almost nops on non-main UIs

Since we always run the inferior in the main console (unless "set
inferior-tty" is in effect), when some UI other than the main one
calls target_terminal_inferior/target_terminal_inferior, then we only
register/unregister the UI's input from the event loop, but leave the
main UI's terminal settings as is.

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* target.c (target_terminal_inferior): Bail out after
	unregistering input_fd if not on the main UI.
	(target_terminal_ours): Bail out after registering input_fd if not
	on the main UI.
	(target_terminal_ours_for_output): Bail out if not on the main UI.