rtadvd(8): support PREF64 (RFC 8781)

PREF64 allows a router to advertise the network's NAT64 prefix, allowing
clients to auto-configure CLAT. This makes it possible to deploy
IPv6-only or IPv6-mostly client access networks without the need for
DNS64.

Reviewed by: imp, glebius (prior suggetions done)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1206