Tell the user if no updates are available after apt update

Thanks to Jakub Wilk for the suggestion.

Closes: #751388
fix autopkgtest tests
fix test-apt-ftparchive-cachedb-lp1274466 and apt-internal-solver tests
Show warning if a repository without signature if found

Show a warning if a repository without a signature is downloaded
to make clear that this is a risky operation.

Closes: #749795
Do not allow fallback from a authenticated repository to a unauthenticated

If a repository was signed before and is no longer signed, stop the
update for the repository and revert to the last known good state.
improve wording of unauthenticated repo warning (thanks to Jakub Wilk)