Revise EVFILT_EXCEPT filters

Revise EVFILT_EXCEPT filters

Restrict the circumstances where EVFILT_EXCEPT filters trigger:
* when out-of-band data is present and NOTE_OOB is requested.
* when the channel is fully closed and consumer is poll(2).

This should clarify the logic and suppress events that kqueue-based
poll(2) does not except.

OK mpi@