MINOR: sock: introduce sock_inet and sock_unix

MINOR: sock: introduce sock_inet and sock_unix

These files will regroup everything specific to AF_INET, AF_INET6 and
AF_UNIX socket definitions and address management. Some code there might
be agnostic to the socket type and could later move to af_xxxx.c but for
now we only support regular sockets so no need to go too far.

The files are quite poor at this step, they only contain the address
comparison function for each address family.