Support Discovering PREF64 in Router Advertisements

[pemensik]

Support for RFC 8781 - Discovering PREF64 in Router Advertisements, would be a nice addition. It can supply address prefix used to translate IPv4 address into working IPv6 address used for 464xlat. If there would be local DNSSEC validating resolver, it needs to know this used range to accept their synthetizations.

[jpirko]

Please feel free to send a patch for this.