If calling llStopMoveToTarget() on an in-world prim, don't send an unnecessary object update if the prim was not moving to target.

This involves making PhysicsActor.PIDActive get as well as set.
On physics components that don't implement this (all characters and some phys engines) we return false.

OpenSim/Region/Physics/BasicPhysicsPlugin/BasicPhysicsActor.cs

@@ -238,6 +238,7 @@ namespace OpenSim.Region.Physics.BasicPhysicsPlugin
 
         public override bool PIDActive
         {
+            get { return false; }
             set { return; }
         }