Add GNU Screen program module

nixos/modules/module-list.nix

@@ -55,6 +55,7 @@
   ./programs/venus.nix
   ./programs/wvdial.nix
   ./programs/zsh/zsh.nix
+  ./programs/screen.nix
   ./rename.nix
   ./security/apparmor.nix
   ./security/apparmor-suid.nix

nixos/modules/programs/screen.nix

@@ -0,0 +1,30 @@
+{ config, pkgs, ... }:
+
+let
+  inherit (pkgs.lib) mkOption mkIf types;
+  cfg = config.programs.screen;
+in
+
+{
+  ###### interface
+
+  options = {
+    programs.screen = {
+
+      screenrc = mkOption {
+        default = "";
+        description = ''
+          The contents of /etc/screenrc file.
+        '';
+        type = types.lines;
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = mkIf (cfg.screenrc != "") {
+    environment.etc."screenrc".text = cfg.screenrc;
+  };
+
+}