pfctl: Another set skip <group> fix

When retrieving the list of group members we cannot simply use
ifa_lookup(), because it expects the interface to have an IP (v4 or v6)
address. This means that interfaces with no address are not found.
This presents as interfacing being alternately marked as skip and not
whenever the rules are re-loaded.

Happily we only need to fix ifa_grouplookup(). Teach it to also accept
AF_LINK (i.e. interface) node_hosts.

PR: 250994
MFC after: 3 days