pf: fix state export in the face of NAT64

Now that we can NAT64 we can have states where the wire and stack address
families (and protocol) are different. Update the state export code to account
for this.

We keep exporting address family and protocol outside of the key, for backwards
compatibility. This'll return misleading information to userspace in the NAT64
case, but it's assumed that userspace will either understand NAT64 (and thus
look for them in the correct place), or not configure it.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47787