use Future.fork rather than Thread.fork, so that the thread is part of the global thread management

--- a/src/HOL/Tools/Sledgehammer/sledgehammer.ML	Mon Sep 06 13:48:10 2010 +0200
+++ b/src/HOL/Tools/Sledgehammer/sledgehammer.ML	Mon Sep 06 16:50:29 2010 +0200
@@ -452,7 +452,7 @@
             else
               ()
           end
-      in if blocking then run () else Toplevel.thread true (tap run) |> K () end
+      in if blocking then run () else Future.fork run |> K () end
 
 val setup =
   dest_dir_setup