Cygwin: FIFO: improve taking ownership in fifo_reader_thread

Cygwin: FIFO: improve taking ownership in fifo_reader_thread

When a reader takes ownership in fifo_reader_thread, it now goes
directly to the part of the main loop that listens for a connection.
Previously it went back to the beginning of the loop.

Also, if the reader has to delay taking ownership because the previous
owner has not finished updating the shared fifo_client handlers, it
now checks to see if cancel_evt has been set.  Previously it might
have had to spin its wheels unnecessarily only to eventually find that
its thread had been canceled.