Cygwin: FIFO: don't leave a pending listen request

On exit from the listen_client thread, make sure there's no pending
FSCTL_PIPE_LISTEN request.  Otherwise we might get a client connection
after restarting the thread, and we won't have a handle for
communicating with that client.

Remove the retry loop in the case of STATUS_PIPE_LISTENING; that case
shouldn't occur.

Remove the now-unused fc_connecting value from
fifo_client_connect_state.

winsup/cygwin/fhandler.h

@@ -1240,7 +1240,6 @@ public:
 enum fifo_client_connect_state
   {
    fc_unknown,
-   fc_connecting,
    fc_connected,
    fc_invalid
   };