Do not allow a script to attach a prim if its being sat upon.

This prevents a stack overflow where a get position on the avatar will refer to the attachment which will in turn refer back to the avatar.
This required recording of all sitting avatars on a prim which is done separately from recording the sit target avatar.
Recording HashSet is null if there are no sitting avatars in order to save memory.

OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs

@@ -3401,6 +3401,20 @@ namespace OpenSim.Region.Framework.Scenes
             return count;
         }
 
+        /// <summary>
+        /// Gets the number of sitting avatars.
+        /// </summary>
+        /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
+        /// <returns></returns>
+        public int GetSittingAvatarsCount()
+        {
+             int count = 0;
+
+             Array.ForEach<SceneObjectPart>(m_parts.GetArray(), p => count += p.GetSittingAvatarsCount());
+
+             return count;
+        }
+
         public override string ToString()
         {
             return String.Format("{0} {1} ({2})", Name, UUID, AbsolutePosition);