change type for address family to "int"

Let's settle on a single type for all address family values, even if
UNIX is very inconsitent on the precise type otherwise. Given that
socket() is the primary entrypoint for the sockets API, and that uses
"int", and "int" is relatively simple and generic, we settle on "int"
for this.
in-addr-util: remove family_to_string() API

we already have a more complete one with af_to_name(), that is generated
from the header files, no need to duplicate this.