nixos/vnstat: homedir -> statedir

before, a nixos update that didn't trigger the chmod would break vnstat

and use a vnstatd group

nixos/modules/services/monitoring/vnstat.nix

@@ -6,21 +6,21 @@ let
   cfg = config.services.vnstat;
 in {
   options.services.vnstat = {
-    enable = mkOption {
-      type = types.bool;
-      default = false;
-      description = ''
-        Whether to enable update of network usage statistics via vnstatd.
-      '';
-    };
+    enable = mkEnableOption "update of network usage statistics via vnstatd";
   };
 
   config = mkIf cfg.enable {
-    users.users.vnstatd = {
-      isSystemUser = true;
-      description = "vnstat daemon user";
-      home = "/var/lib/vnstat";
-      createHome = true;
+
+    environment.systemPackages = [ pkgs.vnstat ];
+
+    users = {
+      groups.vnstatd = {};
+
+      users.vnstatd = {
+        isSystemUser = true;
+        group = "vnstatd";
+        description = "vnstat daemon user";
+      };
     };
 
     systemd.services.vnstat = {
@@ -33,7 +33,6 @@ in {
         "man:vnstat(1)"
         "man:vnstat.conf(5)"
       ];
-      preStart = "chmod 755 /var/lib/vnstat";
       serviceConfig = {
         ExecStart = "${pkgs.vnstat}/bin/vnstatd -n";
         ExecReload = "${pkgs.procps}/bin/kill -HUP $MAINPID";
@@ -52,7 +51,10 @@ in {
         RestrictNamespaces = true;
 
         User = "vnstatd";
+        Group = "vnstatd";
       };
     };
   };
+
+  meta.maintainers = [ maintainers.evils ];
 }