actually closer file reader;

--- a/src/Pure/System/isabelle_system.scala	Tue Dec 22 17:25:41 2009 +0100
+++ b/src/Pure/System/isabelle_system.scala	Tue Dec 22 17:59:59 2009 +0100
@@ -58,6 +58,7 @@
       buf.append(c.toChar)
       c = reader.read
     }
+    reader.close
     buf.toString
   }
 }