Authentication modules are supposed to override res_users.check_credentials()
in order to plug in their own mechanism, without actually modifying the
behavior of res_users.check(), res_users.authenticate() or
res_users._login().
auth_openid was incorrectly overriding check() instead of
check_credentials(), and unnecessarily accessing private
attributes of res_users. Fixing the implementation of auth_openid
to follow the API means we can completely make those attributes
private.