MINOR: protocol: add a new proto_fam structure for protocol families

MINOR: protocol: add a new proto_fam structure for protocol families

We need to specially handle protocol families which regroup common
functions used for a given address family. These functions include
bind(), addrcmp(), get_src() and get_dst() for now. Some fields are
also added about the address family, socket domain (protocol family
passed to the socket() syscall), and address length.

These protocol families are referenced from the protocols but not yet
used.