BulletSim: add implementation of 'physSetLinksetType' and 'physGetLinksetType'

and processing routines in BulletSim.
Add linkset rebuild/conversion routine in BSLinkset.

OpenSim/Region/Physics/Manager/PhysicsScene.cs

@@ -338,7 +338,8 @@ namespace OpenSim.Region.Physics.Manager
         // Extendable interface for new, physics engine specific operations
         public virtual object Extension(string pFunct, params object[] pParams)
         {
-            throw new NotImplementedException();
+            // A NOP if the extension thing is not implemented by the physics engine
+            return null;
         }
     }
 }