Add systemcmd.

--- a/src/Pure/proof_general.ML	Thu Aug 19 00:47:15 2004 +0200
+++ b/src/Pure/proof_general.ML	Thu Aug 19 01:20:17 2004 +0200
@@ -1127,6 +1127,7 @@
 			      | None => raise PGIP ("closefile when no file is open!"))
      | "abortfile"      => (currently_open_file := None) (* perhaps error for no file open *)
      | "changecwd"      => ThyLoad.add_path (xmlattr attrs dirname_attr)
+     | "systemcmd"	=> isarscript data
      (* unofficial command for debugging *)
      | "quitpgip" => raise PGIP_QUIT  
      | _ => raise PGIP ("Unrecognized PGIP element: " ^ elem)))