hip-2: update security for valid addresses

[pinheadmz]

We discussed this in dev chat, that hip2 names should always be prefixed with @ so the user (and wallet software) knows its not intended to be a valid address. Also I think if the string is a valid address (even with the @ included) then hip2 resolution should be aborted.

[Falci]

We have some sort of consensus about it already. I guess we can jump to a PR and comment there.