privoxy service: remove static uid

The service owns no data, having a static uid serves no purpose.

This frees up uid/gid 32

nixos/modules/misc/ids.nix

···
       #utmp = 29; # unused
       ddclient = 30;
       davfs2 = 31;
-      privoxy = 32;
       #disnix = 33; # unused
       osgi = 34;
       tor = 35;
···
       utmp = 29;
       #ddclient = 30; # unused
       davfs2 = 31;
-      privoxy = 32;
       disnix = 33;
       osgi = 34;
       tor = 35;

nixos/modules/services/networking/privoxy.nix

···
 
   inherit (pkgs) privoxy;
 
-  privoxyUser = "privoxy";
-
   cfg = config.services.privoxy;
 
   confFile = pkgs.writeText "privoxy.conf" ''
···
   ###### implementation
 
   config = mkIf cfg.enable {
-  
-    users.extraUsers = singleton
-      { name = privoxyUser;
-        uid = config.ids.uids.privoxy;
-        description = "Privoxy daemon user";
-      };
+
+    users.users.privoxy = {
+      isSystemUser = true;
+      home = "/var/empty";
+      group = "privoxy";
+    };
+
+    users.groups.privoxy = {};
 
     systemd.services.privoxy = {
       description = "Filtering web proxy";
       after = [ "network.target" "nss-lookup.target" ];
       wantedBy = [ "multi-user.target" ];
-      serviceConfig.ExecStart = "${privoxy}/sbin/privoxy --no-daemon --user ${privoxyUser} ${confFile}";
+      serviceConfig.ExecStart = "${privoxy}/bin/privoxy --no-daemon --user privoxy ${confFile}";
 
       serviceConfig.PrivateDevices = true;
       serviceConfig.PrivateTmp = true;