pf: make contents of struct pfsync_state configurable

Make struct pfsync_state contents configurable by sending out new
versions of the structure in separate subheader actions. Both old and
new version of struct pfsync_state can be understood, so replication of
states from a system running an older kernel is possible. The version
being sent out is configured using ifconfig pfsync0 … version XXXX. The
version is an user-friendly string - 1301 stands for FreeBSD 13.1 (I
have checked synchronization against a host running 13.1), 1400 stands
for 14.0.

A host running an older kernel will just ignore the messages and count
them as "packets discarded for bad action".

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D39392