Resolve the socket protocol/address family issues with IPv4 vs. IPv6

When listening, the address family cannot be PF_UNSPEC, so default to PF_INET (IPv4), when not specified.
Not sure why we're setting ai_flags to PF_UNSPEC (copied from syncterm) - that's just 0, but otherwise the PF_ and AI_ flags are not compatible.
Don't set the ai_flags to anything here since they don't seem to change the behavior.