Add back parts of reverted changes that were not concerned with child agent caching.

This adds ScenePresence to IClientAPI.SceneAgent earlier on in the add client process so that its information is available to EventManager.OnNewClient() and OnClientLogin()
Also add a code comment as to why we're caching friend information for child agents.

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -2670,6 +2670,10 @@ namespace OpenSim.Region.Framework.Scenes
                     sp.IsChildAgent ? "child" : "root", sp.Name, RegionInfo.RegionName);
             }
 
+            // We must set this here so that TriggerOnNewClient and TriggerOnClientLogin can determine whether the
+            // client is for a root or child agent.
+            client.SceneAgent = sp;
+
             m_LastLogin = Util.EnvironmentTickCount();
 
             // Cache the user's name