Stun-client uses IPv6 by default

[jech]

In 3b05de6a54fe8f4c9d4091ed98bed3af9bb9470d, I modified stun-client to only use IPv4, since the utility is not useful with IPv6. In 1b149634013db72af3418b247bf8cff98ead54c3, @stv0g broke this, and made stun_client useless again. Interestingly enough, he left in the comment that explains why I restrict the code to IPv4.

Please fix stun_client so that it defaults to IPv4. If that is not possible, please remove the utility, so that people don't wonder what it's for.