disable parallel proofs (again) -- still suffering from instabilites wrt. interrupts;

--- a/src/Pure/System/isabelle_process.ML	Mon Apr 09 21:29:47 2012 +0200
+++ b/src/Pure/System/isabelle_process.ML	Mon Apr 09 23:06:14 2012 +0200
@@ -168,7 +168,7 @@
     val _ = Output.physical_stderr Symbol.STX;
 
     val _ = quick_and_dirty := false;
-    val _ = Goal.parallel_proofs := 1;
+    val _ = Goal.parallel_proofs := 0;
     val _ =
       if Multithreading.max_threads_value () < 2
       then Multithreading.max_threads := 2 else ();