nixos/vnstat: allow overriding package (#448922)

{

  options.services.vnstat = {

    enable = lib.mkEnableOption "update of network usage statistics via vnstatd";

    package = lib.mkPackageOption pkgs "vnstat" { };

  };

  config = lib.mkIf cfg.enable {

    environment.systemPackages = [ pkgs.vnstat ];

    environment.systemPackages = [ cfg.package ];

    users = {

      groups.vnstatd = { };

        "man:vnstat.conf(5)"

      ];

      serviceConfig = {

        ExecStart = "${pkgs.vnstat}/bin/vnstatd -n";

        ExecStart = "${cfg.package}/bin/vnstatd -n";

        ExecReload = "${pkgs.procps}/bin/kill -HUP $MAINPID";

        # Hardening (from upstream example service)