* xterm.c (handle_one_xevent): Don't queue X selection events

* xterm.c (handle_one_xevent): Don't queue X selection events
here, it may be too late if we start queuing after we have already
stored some selection events into the kbd buffer.
(struct selection_event_queue, queue, x_queue_selection_requests)
(x_queue_event, x_unqueue_events, x_start_queuing_selection_requests)
(x_stop_queuing_selection_requests): Remove/move to xselect.c.
(x_catch_errors_unwind): Block input around final XSync.