screen-locker: made xss-lock a systemd service.

Takes advantage of the new `--session` xss-lock parameter to allow
xss-lock be made into a systemd service.

PR #1015

modules/services/screen-locker.nix

@@ -65,12 +65,21 @@ in {
       };
     };
 
-    # xss-lock will run specified screen locker when the session is locked via loginctl
-    # can't be started as a systemd service,
-    # see https://bitbucket.org/raymonad/xss-lock/issues/13/allow-operation-as-systemd-user-unit
-    xsession.initExtra = "${pkgs.xss-lock}/bin/xss-lock ${
-        concatStringsSep " " cfg.xssLockExtraOptions
-      } -- ${cfg.lockCmd} &";
+    systemd.user.services.xss-lock = {
+      Unit = {
+        Description = "xss-lock, session locker service";
+        After = [ "graphical-session-pre.target" ];
+        PartOf = [ "graphical-session.target" ];
+      };
+
+      Install = { WantedBy = [ "graphical-session.target" ]; };
+
+      Service = {
+        ExecStart = concatStringsSep " "
+          ([ "${pkgs.xss-lock}/bin/xss-lock" "-s \${XDG_SESSION_ID}" ]
+            ++ cfg.xssLockExtraOptions ++ [ "-- ${cfg.lockCmd}" ]);
+      };
+    };
   };
 
 }