Implement certificate based authentication.
Reorganize deb822 extensions and move dud.py and changes.py to debile/master.

Except for the deb822 extensions dud.py and changes.py are only useful on the
master, and I want to add master-only stuff to them.
Imporve pgp keyring handling, add ssl keyring handling, update create_builder() xmlrpc method, and add create_user() xmlrpc method.
Check that the subject of a certificate matches the user/builder we are adding