Cygwin: FIFO: avoid crashes when cloning a client

Cygwin: FIFO: avoid crashes when cloning a client

fhandler_fifo::clone called fhandler_base::clone on each client
fhandler.  But those fhandlers are actually fhandler_fifo objects, so
when fhandler_base::clone calls copyto, it's actually
fhandler_fifo::copyto that gets called.  This can lead to mysterious
crashes.

Fix this by simply calling clone (which translates to
fhandler_fifo::clone) on each client fhandler.