nixos: system: add polkit

One nice thing is that it enables the prompts when using `systemctl`,
instead of requiring `sudo`.

modules/nixos/system/default.nix

@@ -10,6 +10,7 @@
     ./nix
     ./packages
     ./podman
+    ./polkit
     ./printing
     ./users
   ];

modules/nixos/system/polkit/default.nix

@@ -0,0 +1,16 @@
+# Polkit settings
+{ config, lib, ... }:
+let
+  cfg = config.my.system.polkit;
+in
+{
+  options.my.system.polkit = with lib; {
+    enable = my.mkDisableOption "polkit configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    security.polkit = {
+      enable = true;
+    };
+  };
+}