Cygwin: FIFO: simplify the listen_client_thread code

Cygwin: FIFO: simplify the listen_client_thread code

Always return 0; no one is doing anything with the return value
anyway.

Remove the return value from stop_listen_client.

Make the connection event auto-reset, so that we don't have to reset
it later.

Simplify the process of connecting a bogus client when thread
termination is signaled.

Make some failures fatal.

Remove the unnecessary extra check for thread termination near the end
of listen_client_thread.