MINOR: protocol: add the real address family to the protocol

MINOR: protocol: add the real address family to the protocol

For custom families, there's sometimes an underlying real address and
it would be nice to be able to directly use the real family in calls
to bind() and connect() without having to add explicit checks for
exceptions everywhere.

Let's add a .real_family field to struct proto_fam for this. For now
it's always equal to the family except for non-transferable ones such
as rhttp where it's equal to the custom one (anything else could fit).