pam: Add default configuration for GNU screen.

This is needed in order to properly lock your screen using the C-a C-x
(lockscreen) command _and_ being back to re-login, because the "other" PAM
service/fallback is to deny authentication.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

modules/security/pam.nix

@@ -239,6 +239,7 @@ in
         { name = "vlock"; }
         { name = "xlock"; }
         { name = "xscreensaver"; }
+        { name = "screen"; }
       ];
 
   };