add apt-key support for armored GPG key files (*.asc)

Having binary files in /etc is kinda annoying – not that the armored
files are much better – but it is hard to keep tabs on which format the
file has ("simple" or "keybox") and different gnupg versions have
different default binary formats which can be confusing for users to
work with (beside that it is binary).

Adding support for this now will enable us in some distant future to
move to armored later on, much like we added trusted.gpg.d years before
the world picked it up.