Cygwin: FIFO: stop using overlapped I/O

Cygwin: FIFO: stop using overlapped I/O

Make fhandler_fifo a derived class of fhandler_base instead of
fhandler_base_overlapped.

Replace the create_pipe macro, which is based on
fhandler_pipe::create, by new create_pipe and open_pipe methods.
These use NT functions instead of Win32 functions.  Replace fifo_name
by get_pipe_name, which returns a pointer to a UNICODE_STRING.

Remove the fnevent macro, which would now be needed only once.

Add a raw_write method, adapted from fhandler_base::raw_write.

Adapt all functions to the changes above.