use apt-key adv (+ gnupg) instead of gpgv for verify

apt-key does the keyring merge as we need it, so we just call it instead
of reimplementing it to do the merging before gpgv. This means we don't
use gpgv anymore (we never depended on it explicitly - bad style), but
it also means that the message in apt-cdrom add is a bit less friendly
as it says loudly "untrusted key", but for a one-time command its okay.