more robust Managed_Process.kill: check after sending signal;

--- a/src/Pure/System/isabelle_system.scala	Wed Sep 22 00:45:42 2010 +0200
+++ b/src/Pure/System/isabelle_system.scala	Wed Sep 22 12:52:35 2010 +0200
@@ -223,7 +223,10 @@
     private val pid = stdout.readLine
 
     private def kill(signal: String): Boolean =
-      execute(true, "kill", "-" + signal, "-" + pid).waitFor == 0
+    {
+      execute(true, "kill", "-" + signal, "-" + pid).waitFor
+      execute(true, "kill", "-0", "-" + pid).waitFor == 0
+    }
 
     private def multi_kill(signal: String): Boolean =
     {