[dns] Unified definition of a direct and a reverse resolution for the canonical name of a host

Reverse resolution of IPs is important. Any host should have one (and surely only one) canonical name, i.e. a domain that resolves to the IP, whose reverse resolution gives the canonical name.

Following the effort of unification of a single source of data that pursues this collection for users and zones, it would be good, when we manage the resverse resolution of our IPs, to design a way to define in a single declaration the direct resolution and the reverse one for the canonical name and the IP of a host.