Obfs4: no udp no ipv6

• this PR ensures that obfs4 is never used with UDP and/or IPv6 addresses.

The former is not allowed because shapeshifter-dispatcher doesn't properly run in conjunction with openvpn. The latter is impossible because there's no net_gateway equivalent for ipv6 routes yet https://community.openvpn.net/openvpn/ticket/1161