Adding LinkUp/LinkDown when a region is started or stoped.

Those two events could be used to compute the region availibility

OpenSim/Region/Framework/Interfaces/ISnmpModule.cs

@@ -15,4 +15,9 @@ public interface ISnmpModule
     void Major(string Message, Scene scene);
     void ColdStart(int step , Scene scene);
     void Shutdown(int step , Scene scene);
+	//
+	// Node Start/stop events
+	//
+    void LinkUp(Scene scene);
+    void LinkDown(Scene scene);
 }