Fix llGetLinkKey() to return the last sat avatar as the last link number.

As per http://wiki.secondlife.com/wiki/LlGetLinkKey
This is done by keeping a scene-object wide list of sitters.
This also fixes bugs in this function where linknums 0 and 1 weren't treated properly if there were sitting avatars on a single prim.
This also fixes a minor race condition for multiple concurrent sitters on a prim with no current sitters by locking on the object-wide list rather than individual sop lists
Addresses http://opensimulator.org/mantis/view.php?id=6477

OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs

@@ -647,6 +647,18 @@ namespace OpenSim.Region.Framework.Scenes
         /// </remarks>
         public UUID FromFolderID { get; set; }
 
+        /// <summary>
+        /// IDs of all avatars sat on this scene object.
+        /// </summary>
+        /// <remarks>
+        /// We need this so that we can maintain a linkset wide ordering of avatars sat on different parts.
+        /// This must be locked before it is read or written.
+        /// SceneObjectPart sitting avatar add/remove code also locks on this object to avoid race conditions.
+        /// No avatar should appear more than once in this list.
+        /// Do not manipulate this list directly - use the Add/Remove sitting avatar methods on SceneObjectPart.
+        /// </remarks>
+        protected internal List<UUID> m_sittingAvatars = new List<UUID>();
+
         #endregion
 
 //        ~SceneObjectGroup()
@@ -3563,6 +3575,20 @@ namespace OpenSim.Region.Framework.Scenes
             return count;
         }
 
+        /// <summary>
+        /// Get a copy of the list of sitting avatars on all prims of this object.
+        /// </summary>
+        /// <remarks>
+        /// This is sorted by the order in which avatars sat down.  If an avatar stands up then all avatars that sat
+        /// down after it move one place down the list.
+        /// </remarks>
+        /// <returns>A list of the sitting avatars.  Returns an empty list if there are no sitting avatars.</returns>
+        public List<UUID> GetSittingAvatars()
+        {
+            lock (m_sittingAvatars)
+                return new List<UUID>(m_sittingAvatars);
+        }
+
         /// <summary>
         /// Gets the number of sitting avatars.
         /// </summary>
@@ -3570,11 +3596,8 @@ namespace OpenSim.Region.Framework.Scenes
         /// <returns></returns>
         public int GetSittingAvatarsCount()
         {
-             int count = 0;
-
-             Array.ForEach<SceneObjectPart>(m_parts.GetArray(), p => count += p.GetSittingAvatarsCount());
-
-             return count;
+            lock (m_sittingAvatars)
+                return m_sittingAvatars.Count;
         }
 
         public override string ToString()
@@ -3583,7 +3606,7 @@ namespace OpenSim.Region.Framework.Scenes
         }
 
         #region ISceneObject
-        
+
         public virtual ISceneObject CloneForNewScene()
         {
             SceneObjectGroup sog = Copy(false);