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