nixos/screen: add `pkgs.screen` to the system closure

nixos/doc/manual/release-notes/rl-1809.xml

···
      <literal>kubectl delete clusterrolebinding kubernetes-dashboard</literal>
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The <varname>programs.screen</varname> module provides allows to configure
+     <literal>/etc/screenrc</literal>, however the module behaved fairly counterintuitive as
+     the config exists, but the package wasn't available. Since 18.09 <literal>pkgs.screen</literal>
+     will be added to <literal>environment.systemPackages</literal>.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/programs/screen.nix

···
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 let
   inherit (lib) mkOption mkIf types;
···
 
   config = mkIf (cfg.screenrc != "") {
     environment.etc."screenrc".text = cfg.screenrc;
+
+    environment.systemPackages = [ pkgs.screen ];
   };
 
 }