Update pf(4) and pflog(4) to survive basic VNET testing, which includes

Update pf(4) and pflog(4) to survive basic VNET testing, which includes
proper virtualisation, teardown, avoiding use-after-free, race conditions,
no longer creating a thread per VNET (which could easily be a couple of
thousand threads), gracefully ignoring global events (e.g., eventhandlers)
on teardown, clearing various globally cached pointers and checking
them before use.

Reviewed by:		kp
Approved by:		re (gjb)
Sponsored by:		The FreeBSD Foundation
MFC after:		2 weeks
Differential Revision:	https://reviews.freebsd.org/D6924