Stop NPCs losing attachments when the source avatar takes them off.

This was happening because we were using the source avatar's item IDs in the clone appearance.
Switch to using the asset IDs of attachments instead for NPCs.
The InventoryAccessModule and AttachmentModule had to be changed to allow rezzing of an object without an associated inventory item.
Hopefully goes some way towards resolving http://opensimulator.org/mantis/view.php?id=5653

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -75,9 +75,6 @@ namespace OpenSim.Region.Framework.Scenes
 
         private static readonly ILog m_log = LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType);
 
-        /// <summary>
-        /// What type of presence is this?  User, NPC, etc.
-        /// </summary>
         public PresenceType PresenceType { get; private set; }
 
 //        private static readonly byte[] DEFAULT_TEXTURE = AvatarAppearance.GetDefaultTexture().GetBytes();