MINOR: protocol: properly assign the sock_domain and sock_family

MINOR: protocol: properly assign the sock_domain and sock_family

When we finally split sock_domain from sock_family in 2.3, something
was not cleanly finished. The family is what should be stored in the
address while the domain is what is supposed to be passed to socket().
But for the custom addresses, we did the opposite, just because the
protocol_lookup() function was acting on the domain, not the family
(both of which are equal for non-custom addresses).

This is an API bug but there's no point backporting it since it does
not have visible effects. It was visible in the code since a few places
were using PF_UNIX while others were comparing the domain against AF_MAX
instead of comparing the family.

This patch clarifies this in the comments on top of proto_fam, addresses
the indexing issue and properly reconfigures the two custom families.