kqueue: Revise filterops interface

kqueue: Revise filterops interface

Extend kqueue's filterops interface with new callbacks so that it
becomes easier to use with fine-grained locking. The new interface
delegates the serialization of kn_event access to event sources. Now
kqueue uses filterops callbacks to read or write kn_event. This hides
event sources' locking patterns from kqueue, and allows clean
implementation of atomic read-and-clear for EV_CLEAR, for instance.

There are so many existing filterops instances that converting all of
them in one go is tricky. This patch adds a wrapper mechanism that
kqueue uses when the new callbacks are missing.

The new filterops interface has been influenced by XNU's kqueue.

OK mpi@ semarie@