Draft: OpenPGP public key validator

Implement a custom OpenPGP parser derived from pgpdump, with all the non-interesting bits cut out, and use it to list all the files in a keyring.

The next step here obviously is to also store the raw public key with subkeys and signature packets in the Keyring object and then we are able to dump the keys from the data structure to a temporary keyring.

And then we just need to implement dearmored reading, and we have most of the keyring setup from apt-key.