pf: nat64
Since the IPv6 madness is not enough introduce NAT64 -- which is actually
"af-to" a generic IP version translator for pf(4).
Not everything perfect yet but lets fix these things in the tree.
Insane amount of work done by sperreault@, mikeb@ and reyk@.
Looked over by mcbride@ henning@ and myself at eurobsdcon.
OK mcbride@ and general put it in from deraadt@
Obtained from: OpenBSD, claudio <claudio@openbsd.org>, 97326e01c9
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D47786