Extensive reworking of the "select" interface, so that it can work

Extensive reworking of the "select" interface, so that it can work
with pipes.  We no longer use the NT API calls, since they don't work
at all with pipes; instead we use simpler tests and depend on the
regular arrival of messages to get us out of the blocking state to
poll the other input channels.  Regular message delivery is guaranteed
by the asynchronous timer thread.