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/Helpers.cs

@@ -81,6 +81,24 @@ namespace OpenSim.Region.ScriptEngine.Shared
         }
     }
 
+    /// <summary>
+    /// Used to signal when the script is stopping in co-operation with the script engine 
+    /// (instead of through Thread.Abort()).
+    /// </summary>
+    [Serializable]
+    public class ScriptCoopStopException : Exception
+    {
+        public ScriptCoopStopException()
+        {
+        }
+
+        protected ScriptCoopStopException(
+                SerializationInfo info, 
+                StreamingContext context)
+        {
+        }
+    }
+
     public class DetectParams
     {
         public const int AGENT = 1;