Make UUID.Zero resolve to "Unknown User" in user cache.

This is to avoid massive numbers of 'no user found' logs when user IDs are missing for some reason.
UUID.Zero should not be used for any user ID.

OpenSim/Region/CoreModules/Framework/UserManagement/UserManagementModule.cs

@@ -73,6 +73,8 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
                 Init();
                 m_log.DebugFormat("[USER MANAGEMENT MODULE]: {0} is enabled", Name);
             }
+
+            AddUser(UUID.Zero, "Unknown", "User");
         }
 
         public bool IsSharedModule