kqueue: Invalidate revoked vnodes' knotes on the fly

kqueue: Invalidate revoked vnodes' knotes on the fly

When a tty device is revoked, the associated knotes should be
invalidated. Otherwise the user processes can keep on receiving
events from the device.

It appears tricky to do the invalidation as part of revocation
in a way that does not allow unwanted event registration or clutter
the tty code. For now, make the knotes invalid lazily before delivery.

OK mpi@