prefer explicit thread;

--- a/src/Pure/System/cygwin_init.scala	Fri Sep 06 18:14:50 2013 +0200
+++ b/src/Pure/System/cygwin_init.scala	Fri Sep 06 18:15:25 2013 +0200
@@ -54,7 +54,7 @@
         case None =>
         case Some(0) =>
           visible = false
-          default_thread_pool.submit(() => start)
+          Simple_Thread.fork("Isabelle") { start }
         case Some(rc) =>
           sys.exit(rc)
       }