Implement RFC 8781 PREF64 router advertisement option.

Implement RFC 8781 PREF64 router advertisement option.

With this clients can learn the presence and used prefix for Network
Address and Protocol Translation between IPv6 and IPv4 (NAT64).

Apparently there is support in mobile devices as well as in macOS.

This option, together with the the dhcp "IPv6-only preferred"
option (108) enables the Customer-side transLATor (CLAT) on macOS so
IPv4 literals can be used in IPv6-only networks.

Input & OK kn