Make filt_dead() selectively inactive with EVFILT_EXCEPT

Make filt_dead() selectively inactive with EVFILT_EXCEPT

When a knote uses the dead event filter, the knote's file descriptor is
not supposed to point to an object with pending out-of-band data. Make
the knote inactive so that userspace will not receive a spurious event.
However, kqueue-based poll(2) should still receive HUP notifications.

This lets the system use dead_filtops with less strings attached
relative to the filter type.