(sys_select): Call MsgWaitForMultipleObjects instead

(sys_select): Call MsgWaitForMultipleObjects instead
of WaitForMultipleObjects when user input is allowed, so we can
handle incoming window messages.  Call drain_message_queue when
there are messages waiting; this ensures that windows created
indirectly from the lisp thread get processed properly, and don't
hang other applications by failing to respond to broadcasts.