This patch makes `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".