This patch introduces ui_out_redirect_pop.  All uses of
make_cleanup_ui_out_redirect_pop are replaced with this new class.

ChangeLog
2017-09-09  Tom Tromey  <tom@tromey.com>

	* mi/mi-interp.c (mi_user_selected_context_changed): Use
	ui_out_redirect_pop.
	* guile/scm-ports.c (ioscm_with_output_to_port_worker): Use
	ui_out_redirect_pop.
	* utils.c (do_ui_out_redirect_pop)
	(make_cleanup_ui_out_redirect_pop): Remove.
	* top.c (execute_command_to_string): Use ui_out_redirect_pop.
	* utils.h (make_cleanup_ui_out_redirect_pop): Remove.
	* ui-out.h (ui_out_redirect_pop): New class.