users-groups service: add autoSubUidGidRange option

Previously we allocated subuids automatically for all normal users.
Make this explicitly configurable, so that one can use this for system
users too (or explicitly disable for normal users). Also don't allocate
automatically by default if a user already has ranges specified statically.

nixos/modules/config/users-groups.nix

@@ -204,6 +204,16 @@ let
         '';
       };
 
+      autoSubUidGidRange = mkOption {
+        type = types.bool;
+        default = false;
+        example = true;
+        description = ''
+          Automatically allocate subordinate user and group ids for this user.
+          Allocated range is currently always of size 65536.
+        '';
+      };
+
       createHome = mkOption {
         type = types.bool;
         default = false;
@@ -320,6 +330,9 @@ let
         (mkIf (!cfg.mutableUsers && config.initialHashedPassword != null) {
           hashedPassword = mkDefault config.initialHashedPassword;
         })
+        (mkIf (config.isNormalUser && config.subUidRanges == [] && config.subGidRanges == []) {
+          autoSubUidGidRange = mkDefault true;
+        })
       ];
 
   };
@@ -419,7 +432,7 @@ let
       { inherit (u)
           name uid group description home createHome isSystemUser
           password passwordFile hashedPassword
-          isNormalUser subUidRanges subGidRanges
+          autoSubUidGidRange subUidRanges subGidRanges
           initialPassword initialHashedPassword;
         shell = utils.toShellPath u.shell;
       }) cfg.users;