Add f_modify and f_process callbacks to socket filterops.

Add f_modify and f_process callbacks to socket filterops.

This makes kqueue use the extended callback interface with socket event
filters. Now one level of nested kernel locking is avoided, and the
callbacks run without splhigh().

The filterops no longer check NOTE_SUBMIT, and use a fixed locking
pattern instead. The f_event routines are always called with solock(),
whereas f_modify and f_process are always called without the lock.

OK mpi@