Print warning for unauthenticated repositories
Do not allow going from authenticated to unauthenticated repo

Also rework the way we load the Release file, so it only after
Release.gpg verified the Release file. The rational is that we
never want to load untrusted data into our parsers. Only stuff
verified with gpg or by its hashes get loaded. To load untrusted
data you now need to use apt-get update --allow-unauthenticated.