KEYS: keyring: Provide key preparsing

Provide key preparsing in the keyring so that we can make preparsing
mandatory.  For keyrings, however, only an empty payload is permitted.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Jeff Layton <jlayton@primarydata.com>