This can happen if for example a secret is used in the initrd, which
materializes it as a directory, which then causes agenix to silently
create an incorrect link when switching to stage2. This ensures that
agenix will abort with an error.
Why
===
* Someone said age works better with password protected keys,
requiring entering the password less often.
* We switched to rage from age in
07ce686870
because it was limiting recipients to 20. This was fixed
https://github.com/FiloSottile/age/issues/139
What changed
===
* Switch from rage back to age (the reference implementation) in all
the spots
* Update the docs to show how to switch back to Rage
* Skip keys that are empty files, which fixes the integration test.
