Fix auth_cfg_t.clone() for single-valued auth rules

Fix auth_cfg_t.clone() for single-valued auth rules

By using the default list enumerator and adding the rules with the public
add() method, clones of auth_cfg_t objects would return the values for
single-valued auth rules in the wrong order (i.e. the oldest instead of the
newest value was returned).  Using the internal enumerator (which the comment
already suggested) fixes this, but the clone will not be a full clone as
it does not contain any old values for single-valued auth rules.  Since
these will never be used anyway, this should be fine.