Set POLLHUP even if no valid events were specified as per POSIX.

Set POLLHUP even if no valid events were specified as per POSIX.
Since we use the poll backend for select(2), care must be taken not
to set the fd's bit in writefds in this case.  A kernel-only flag,
POLLNOHUP, is used by selscan() to tell the poll backend not to
return POLLHUP on EOF.  This is currently only used by fifo_poll().
The fifofs regress now passes.  OK guenther@