netaddr_t !(AF_INET || AF_INET6) handling

While looking isc_netaddr_fromsockaddr() code, I found out that it can handle AF_UNIX sockets as well, but most of the code that process the resulting struct isc_netaddr only takes care of AF_INET or AF_INET6 classes in a way that would process of reading arbitrary bytes from struct sockaddr_un path.

The code using netaddr_t should be changed to properly check for values that makes sense, e.g. from:

if (family == AF_INET6) {
 // IPv6 code
} else {
 // IPv4 code
}

to

if (family == AF_INET6) {
  // IPv6 code
} else if (family == AF_INET) {
  // IPv4 code
} else {
  // handle unknown AF_
}

That would also allow us to remove the hard INSIST(0) from default branch in isc_netaddr_fromsockaddr and change it to warning + setting family to AF_UNSPEC on an unknown socket.