CLEANUP: protocol: remove family-specific fields from struct protocol

CLEANUP: protocol: remove family-specific fields from struct protocol

This removes the following fields from struct protocol that are now
retrieved from the protocol family instead: .sock_family, .sock_addrlen,
.l3_addrlen, .addrcmp, .bind, .get_src, .get_dst.

This also removes the UDP-specific udp{,6}_get_{src,dst}() functions
which were referenced but not used yet. Their goal was only to remap
the original AF_INET* addresses to AF_CUST_UDP*.

Note that .sock_domain is still there as it's used as a selector for
the protocol struct to be used.