Prefer ed25519 over rsa signatures

[dodmi]

This changes the signature sort behavior, such that an ed25519 signature will be preferred over a corresponding rsa signature.

For any ed25519 signature, there's a corresponding rsa signature for backward compatibility;
in the case that weak rsa keys are handled as warning and the ed25519 signature also produces a warning, it might happen, that the rsa signature is ranked first and therefore a weak key warning appeared in spite of a valid ed25519 signature is in place.