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

`xsslock` (which was originally packaged in 6cb1d1aaaf)
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/programs/xss-lock.nix

@@ -0,0 +1,26 @@
+{ 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}";
+    };
+  };
+}