Implement co-operative script termination if termination comes during a script wait event (llSleep(), etc.)

This makes use of EventWaitHandles since various web references indicate that Thread.Interrupt() can also cause runtime instability.
If co-op termination is enabled, then termination sets the wait handle instead of waiting for a timeout before possibly aborting the thread.
This allows the script to cleanly terminate if it's in a llSleep/LL function delay or the next time it enters such a wait without any timeout period.
Co-op termination is not yet testable since checking for termination request within loops that never trigger a wait is not yet implemented.
This commit, unlike 1b5c41c, passes the wait handle as an extra parameter through IScript.Initialize() instead of passing IScriptInstance itself.

OpenSim/Region/ScriptEngine/XEngine/XEngine.cs

@@ -1716,9 +1716,14 @@ namespace OpenSim.Region.ScriptEngine.XEngine
             IScriptInstance instance = GetInstance(itemID);
 
             if (instance != null)
+            {
                 instance.Stop(m_WaitForEventCompletionOnScriptStop);
+            }
             else
+            {
+//                m_log.DebugFormat("[XENGINE]: Could not find script with ID {0} to stop in {1}", itemID, World.Name);
                 m_runFlags.AddOrUpdate(itemID, false, 240);
+            }
         }
 
         public DetectParams GetDetectParams(UUID itemID, int idx)