nixos/ddclient: added assertions for passwordFile and secretsFile

      lib.optional (cfg.use != "")

        "Setting `use` is deprecated, ddclient now supports `usev4` and `usev6` for separate IPv4/IPv6 configuration.";

    assertions = [

      {

        assertion = !((cfg.passwordFile != null) && (cfg.secretsFile != null));

        message = "You cannot use both services.ddclient.passwordFile and services.ddclient.secretsFile at the same time.";

      }

      {

        assertion = !(cfg.protocol == "nsupdate") || (cfg.passwordFile == null && cfg.secretsFile == null);

        message = "You cannot use services.ddclient.passwordFile and or services.ddclient.secretsFile when services.ddclient.protocol is \"nsupdate\".";

      }

    ];

    systemd.services.ddclient = {

      description = "Dynamic DNS Client";

      wantedBy = [ "multi-user.target" ];