Now that we have a_fflag in struct vop_poll_args we can handle

Now that we have a_fflag in struct vop_poll_args we can handle
things like POLLOUT on a read-only fd more sensibly.  Previously,
any poll events could be used regardless of the actual file flags
of the descriptor.  The new fifo_poll() is effectively a pared down
soo_poll() with slightly different semantics to batter match FIFOs.
OK and feedback from guenther@