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/Shared/Tests/LSL_ApiTest.cs

@@ -67,7 +67,7 @@ namespace OpenSim.Region.ScriptEngine.Shared.Tests
             engine.AddRegion(scene);
 
             m_lslApi = new LSL_Api();
-            m_lslApi.Initialize(new ScriptInstance(engine, part, null, 0, false, int.MaxValue));
+            m_lslApi.Initialize(engine, part, null, null);
         }
 
         [Test]