Don't try to save changed attachment states when an NPC with attachments is removed from the scene.

This is done by introducing a PresenceType enum into ScenePresence which currently has two values, User and Npc.
This seems better than a SaveAttachments flag in terms of code comprehension, though I'm still slightly uneasy about introducing these semantics to core objects

OpenSim/Region/ClientStack/Linden/UDP/LLClientView.cs

@@ -692,7 +692,7 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
         public virtual void Start()
         {
-            m_scene.AddNewClient(this);
+            m_scene.AddNewClient(this, PresenceType.User);
 
             RefreshGroupMembership();
         }