users: add option to run systemctl --failed on login (#144)

modules/users.nix

@@ -0,0 +1,14 @@
+{ config, lib, ... }:
+
+{
+  options = {
+    users.showFailedUnitsOnLogin = lib.mkEnableOption "show failed systemd units on interactive login";
+  };
+
+  config = lib.mkIf config.users.showFailedUnitsOnLogin {
+    environment.interactiveShellInit = /* sh */ ''
+      # raise some awareness towards failed services
+      systemctl --no-pager --failed || true
+    '';
+  };
+}