restore simple_cnf/add_single API

restore simple_cnf/add_single API

Restore an API that vanished at some point despite there still
being users.

This is commit fa747d25a7c47a1ceef165c4e1667b9755c9da0d on
autotest/i2n. The original commit message in full:

    In

        commit 5f6735c41fc769ac9ba6ee62754ab2b7da3d68d2
        Author: Christian Herdtweck <christian.herdtweck@intra2net.com>
        Date:   Wed Apr 5 18:14:02 2017 +0200

            Extend SimpleCnf.add_single to SimpleCnf.add

    this ApI was folded into .add() without providing an overload for
    downstream consumers. Provide the symbol again as a thin wrapper
    around .add() and hope for the behavior to remain unaffected.