MEDIUM: protocol: rely on AF_CUST_ABNS family to recognize ABNS sockets

MEDIUM: protocol: rely on AF_CUST_ABNS family to recognize ABNS sockets

Now that we can easily distinguish regular UNIX socket from ABNS sockets
by simply looking at the address family, stop looking at the first byte
from addr->sun_path to guess if the socket is an ABNS one or not. Looking
at the family is straightforward and will allow to differentiate between
upcoming ABNSZ and ABNS (where looking at the first byte from path won't
help anymore).