Prevent kevent(2) use of EVFILT_EXCEPT with FIFOs and pipes

Prevent kevent(2) use of EVFILT_EXCEPT with FIFOs and pipes

Currently, the only intended direct usage of the EVFILT_EXCEPT filter
is with NOTE_OOB to detect out-of-band data in ptys and sockets.
NOTE_OOB does not apply to FIFOs or pipes. Prevent the user from
registering the filter with these file types. The filter code is for
the kernel's internal use.

OK mpi@