Cygwin: FIFO: code simplifications.

There's no longer a need to consider the connect_evt after fork/exec.
After stopping the listen_client thread, all client handlers should be
in the fc_connected or fc_invalid states, so their connect_evt members
won't be used again.

Also remove code in fhandler_fifo::dup that just repeats things
already done in stop_listen_client.