nano: support system-wide nanorc

This patch does two things
1. builds nano with sysconfdir=/etc; and
2. adds an option programs.nano.nanorc

nixos/modules/module-list.nix

@@ -52,6 +52,7 @@
   ./programs/blcr.nix
   ./programs/environment.nix
   ./programs/info.nix
+  ./programs/nano.nix
   ./programs/screen.nix
   ./programs/shadow.nix
   ./programs/shell.nix

nixos/modules/programs/nano.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.programs.nano;
+in
+
+{
+  ###### interface
+
+  options = {
+    programs.nano = {
+
+      nanorc = lib.mkOption {
+        type = lib.types.lines;
+        default = "";
+        description = ''
+          The system-wide nano configuration.
+          See <citerefentry><refentrytitle>nanorc</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+        '';
+        example = ''
+          set nowrap
+          set tabstospaces
+          set tabsize 4
+        '';
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = lib.mkIf (cfg.nanorc != "") {
+    environment.etc."nanorc".text = cfg.nanorc;
+  };
+
+}

pkgs/applications/editors/nano/default.nix

@@ -13,6 +13,8 @@ stdenv.mkDerivation (rec {
 
   buildInputs = [ ncurses gettext ];
 
+  configureFlags = "sysconfdir=/etc";
+
   meta = {
     homepage = http://www.nano-editor.org/;
     description = "A small, user-friendly console text editor";