pf: Use different address family for source and redirection address

The function pf_map_addr() and source tracking operate on a single
address family. This made sense before introducing address family
translation. When combining af-to with route-to or with sticky-address,
the next-hop or the NAT address are of different address family than
the source address. For example in NAT64 scenaro an IPv6 source address
is translated to an IPv4 address and routed over IPv4 gateway.

Make source nodes dual-AF, that is have a separate source AF and
redirection AF. Store route AF in struct pf_kstate, export it to pfctl.
When loading rules with redirection pools with pfctl store address
family of each address. When printing states don't deduce next-hop's
address family from af-to, use the one stored in state.

Reviewed by: kp
Approved by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D51659