lib: copy module from NixOS

Importing the module directly from NixOS causes the documentation to
break, in particular the "Declared by" section.

Fixes #405

modules/misc/lib.nix

@@ -0,0 +1,14 @@
+{ lib, ... }:
+
+{
+  options = {
+    lib = lib.mkOption {
+      type = lib.types.attrsOf lib.types.attrs;
+      default = {};
+      description = ''
+        This option allows modules to define helper functions,
+        constants, etc.
+      '';
+    };
+  };
+}

modules/modules.nix

@@ -19,6 +19,7 @@ let
     ./manual.nix
     ./misc/fontconfig.nix
     ./misc/gtk.nix
+    ./misc/lib.nix
     ./misc/news.nix
     ./misc/nixpkgs.nix
     ./misc/pam.nix
@@ -98,7 +99,6 @@ let
     ./xresources.nix
     ./xsession.nix
     <nixpkgs/nixos/modules/misc/assertions.nix>
-    <nixpkgs/nixos/modules/misc/lib.nix>
     <nixpkgs/nixos/modules/misc/meta.nix>
   ]
   ++