Introduce make_cleanup_restore_current_ui

Introduce make_cleanup_restore_current_ui

Just a tidy, no functional changes.

gdb/ChangeLog:
2016-09-06  Pedro Alves  <palves@redhat.com>

	* event-top.c (restore_ui_cleanup): Now static.
	(make_cleanup_restore_current_ui): New function.
	(switch_thru_all_uis_init): Use it.
	* infcall.c (call_thread_fsm_should_stop): Use it.
	* infrun.c (fetch_inferior_event): Use it.
	* top.c (new_ui_command): Use it.
	* top.h (restore_ui_cleanup): Delete declaration.
	(make_cleanup_restore_current_ui): New declaration.