Add an OpenPGP keyring backed database backend

Allow the use of an OpenPGP keyring (like keyring.gpg from GnuPG v1) as
a read-only database backend. This is just a collection of keys
concatenated together. The file is mmaped on load and parsed for keys,
then simple linear searches are done for keyids/fingerprints.
Performance with large numbers of keys will not be good.