in "overlord" mode: ignore problem prefix specified in the .thy file

--- a/src/HOL/Tools/ATP_Manager/atp_systems.ML	Thu Apr 29 19:07:36 2010 +0200
+++ b/src/HOL/Tools/ATP_Manager/atp_systems.ML	Thu Apr 29 19:08:02 2010 +0200
@@ -141,8 +141,8 @@
     fun prob_pathname nr =
       let
         val probfile =
-          Path.basic (the_problem_prefix ^
-                      (if overlord then "_" ^ name else serial_string ())
+          Path.basic ((if overlord then "prob_" ^ name
+                       else the_problem_prefix ^ serial_string ())
                       ^ "_" ^ string_of_int nr)
       in
         if the_dest_dir = "" then File.tmp_path probfile