send: broadcast to all waiting threads;

--- a/src/Pure/Concurrent/mailbox.ML	Sun Sep 07 17:46:37 2008 +0200
+++ b/src/Pure/Concurrent/mailbox.ML	Sun Sep 07 17:46:38 2008 +0200
@@ -37,7 +37,7 @@
     val _ = Mutex.lock lock;
     val _ = change messages (Queue.enqueue msg);
     val _ = Mutex.unlock lock;
-    val _ = ConditionVar.signal cond;
+    val _ = ConditionVar.broadcast cond;
   in () end) ();