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.

OpenSim/Region/ScriptEngine/Interfaces/IScriptInstance.cs

@@ -28,6 +28,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Threading;
 using OpenMetaverse;
 using log4net;
 using OpenSim.Framework;
@@ -180,6 +181,18 @@ namespace OpenSim.Region.ScriptEngine.Interfaces
         void Suspend();
         void Resume();
 
+        /// <summary>
+        /// If true then scripts should look to terminate their threads in co-operation with the script engine rather
+        /// than through Thread.Abort()
+        /// </summary>
+        bool CoopTermination { get; }
+
+        /// <summary>
+        /// Used for script sleeps when we are using co-operative script termination.
+        /// </summary>
+        /// <remarks>null if CoopTermination is not active</remarks>  
+        EventWaitHandle CoopSleepHandle { get; }
+
         /// <summary>
         /// Process the next event queued for this script instance.
         /// </summary>