Provide an option to make the KDC also listen on loopback interfaces for
        datagram requests. Adds an internal symbol to libkrb5 which the KDC
        will need if listening on loopback is enabled.
    The default might be better changed from FALSE to TRUE so that the default
        matches what we do with stream sockets. Or maybe that should be the
        default anyway, with no configuration option.
    FIXME: doesn't add documentation anywhere.