add and use 'apt-key verify' which prefers gpgv over gpg

add and use 'apt-key verify' which prefers gpgv over gpg

gnupg/gnupg2 can do verify just fine of course, so we don't need to use
gpgv here, but it is what we always used in the past, so there might be
scripts expecting a certain output and more importantly the output of
apt-cdrom contains messages from gpg and even with all the settings we
activate to prevent it, it still shows (in some versions) a quiet scary:
"gpg: WARNING: Using untrusted key!" message. Keeping the use of gpgv is
the simplest way to prevent it.

We are increasing also the "Breaks: apt" version from libapt as it
requires a newer apt-key than might be installed in partial upgrades.