instead of async channels. This change allows Fred to work. The
change is needed due to implementation weaknesses at multiple
levels, but mostly because `on-subwindow-event' has to complete
atomically --- or else events are pessimistically discarded, and
async-channel communication never completes atomically. In contrast,
messages can be posted to the built-in message queue for a thread
(because it's built in). Probably the async-channel library should
switch to using the built-in thread queue support.
Merge to 5.1
