Introduced factory method to create JDK's Selector instances.

jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java

@@ -446,10 +446,15 @@ public abstract class SelectorManager extends AbstractLifeCycle implements Dumpa
         protected void doStart() throws Exception
         {
             super.doStart();
-            _selector = Selector.open();
+            _selector = newSelector();
             _state.set(State.PROCESS);
         }
 
+        protected Selector newSelector() throws IOException
+        {
+            return Selector.open();
+        }
+
         @Override
         protected void doStop() throws Exception
         {