Add support for authorized revocations.
New 'keyring-add-revoker' API allows to authorize an external revoker.
'keyring-revoke' API was changed to support authorized revocations.
'keyring-certify' and 'keyring-revoke' API's were generalized to accept
either secret or private keyrings where appropriate. New API's:
'keyring-publish' (public keys extraction), 'keyring-save' (keyring
serialization) and 'keyring-revoked?' (keyring revocation testing).