IPv6 addresses can be unparsable in yaml

IPv6 addresses sometimes end in two trailing colons. For example, in the current output of dig +yaml ns by there's a name server with address "2a05:4800:1:100::". This breaks YAML parsing.

To fix this we need to append "0". I think it would be harmless in all IPv6 address expansions everywhere, but in any case we should always do it when using dns_masterstyle_yaml.