This patch generalizes the concept used in Piano to wake up the event loop
so it can react to something happening on a secondary thread.
Basically, there's a pipe who is always part of the file descriptor set we
pass to select(), and calling wake() simply writes a little to that pipe.
Don't process any more events. We already prepend the remaining events in
this loop to the outer loop if needed.
If there were any more events queued after the exit request, the iteration
code would make an invalid access into 'queued_events'.
Fixes#300.
