MINOR: protocol: add a real family for existing FDs

MINOR: protocol: add a real family for existing FDs

At some places (log fd@XXX, bind fd@XXX) we support using an explicit
file descriptor number, that is placed into the sockaddr for later use.
The problem is that till now it was done with an AF_UNSPEC family, which
is also used for other situations like missing info or rings (for logs).

Let's create an "official" family AF_CUST_EXISTING_FD for this case so
that we are certain the FD can be found in the address when it is set.