users: make the service manager partly configurable

modules/users/default.nix

@@ -153,6 +153,13 @@ let
         services to define for this user.
       '';
     };
+    serviceManager = mkOption {
+      type = types.nullOr (types.enum [ "s6" ]);
+      default = "s6";
+      description = ''
+        which service manager to plumb `services` into.
+      '';
+    };
   };
   userModule = let
     nixConfig = config;

modules/users/s6-rc.nix

@@ -325,7 +325,7 @@ in
       xdg_runtime_dir = "/run/user/${name}";
       scanDir = mkScanDir "${xdg_runtime_dir}/s6/live" compiled;
       liveDir = mkLiveDir compiled;
-    in {
+    in lib.mkIf (config.serviceManager == "s6") {
       fs.".config/s6/live".symlink.target = liveDir;
       fs.".config/s6/scandir".symlink.target = scanDir;
       fs.".config/s6/compiled".symlink.target = compiled;