Rewrite cached key resolution

* src/cryptcontroller.cpp (resolve_through_protocol): New.
(resolve_keys_cached): Use new helper function.

The old code was too much spaghetti and several issues. E.g.
it could select a CMS Signing key with OpenPGP Encryption
keys. Or would not support fallback to S/MIME if OpenPGP
encryption was possible but only an S/MIME signing key available.