Cygwin: FIFO: re-implement duplexers

Cygwin: FIFO: re-implement duplexers

When opening a duplexer, open a client connection to the first client
handler.  Previously we gave the duplexer a bogus write handle, which
was just a duplicate of the first client handler's handle.  This meant
that we had a pipe server with no clients connected, and all I/O
attempts failed with STATUS_PIPE_LISTENING.

Extend the last fcntl change to duplexers.

Remove a now unused fifo_client_handler constructor, as well as the
long unusued method fifo_client_handler::connect.

Don't create the pipe in duplex mode; the server handle will only be
used for reading.