nixos/xss-lock: add module (#40619)

`xsslock` (which was originally packaged in 6cb1d1aaaf02a72329bedf9c6960e54fea6f5c6e)
is a simple screensaver which connects a given screen locker (e.g.
`i3lock`) with `logind`. Whenever `loginctl lock-sessions` is invoked
the locker will be used to lock the screen. This works with its power
management features (e.g. `lid switch`) as well, so the PC can be locked
automatically when the lid is closed.

The module can be used like this:

```
{
services.xserver.enable = true;

programs.xss-lock.enable = true;
programs.xss-lock.lockerCommand = "i3lock";
}
```

nixos/modules/module-list.nix

···
   ./programs/wireshark.nix
   ./programs/xfs_quota.nix
   ./programs/xonsh.nix
+  ./programs/xss-lock.nix
   ./programs/yabar.nix
   ./programs/zsh/oh-my-zsh.nix
   ./programs/zsh/zsh.nix

nixos/modules/programs/xss-lock.nix

···
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.xss-lock;
+in
+{
+  options.programs.xss-lock = {
+    enable = mkEnableOption "xss-lock";
+    lockerCommand = mkOption {
+      example = "xlock";
+      type = types.string;
+      description = "Locker to be used with xsslock";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.user.services.xss-lock = {
+      description = "XSS Lock Daemon";
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
+      serviceConfig.ExecStart = "${pkgs.xss-lock}/bin/xss-lock ${cfg.lockerCommand}";
+    };
+  };
+}

nixos/release.nix

···
   tests.xfce = callTest tests/xfce.nix {};
   tests.xmonad = callTest tests/xmonad.nix {};
   tests.xrdp = callTest tests/xrdp.nix {};
+  tests.xss-lock = callTest tests/xss-lock.nix {};
   tests.yabar = callTest tests/yabar.nix {};
   tests.zookeeper = callTest tests/zookeeper.nix {};
 

nixos/tests/xss-lock.nix

···
+import ./make-test.nix ({ pkgs, lib, ... }:
+
+with lib;
+
+{
+  name = "xss-lock";
+  meta.maintainers = with pkgs.stdenv.lib.maintainers; [ ma27 ];
+
+  machine = {
+    imports = [ ./common/x11.nix ./common/user-account.nix ];
+    programs.xss-lock.enable = true;
+    programs.xss-lock.lockerCommand = "${pkgs.xlockmore}/bin/xlock";
+    services.xserver.displayManager.auto.user = "alice";
+  };
+
+  testScript = ''
+    $machine->start;
+    $machine->waitForX;
+    $machine->waitForUnit("xss-lock.service", "alice");
+
+    $machine->fail("pgrep xlock");
+    $machine->succeed("su -l alice -c 'xset dpms force standby'");
+    $machine->succeed("pgrep xlock");
+  '';
+})