prevent doing filter ops locking for staticly compiled filter ops...
This significantly reduces lock contention when adding/removing knotes
on busy multi-kq system...  Next step is to cache these references per
kq.. i.e. kq refs it once and keeps a local ref count so that the same
refs don't get accessed by many cpus...

only allocate a knote when we might use it...

Add a new flag, _FORCEONESHOT..  This allows a thread to force the
delivery of another event in a safe manner, say waking up an idle http
connection to force it to be reaped...

If we are _DISABLE'ing a knote, don't bother to call f_event on it, it's
disabled, so won't be delivered anyways..

Tested by:	adrian


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274560 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f