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/BulletSPlugin/BSCharacter.cs

@@ -614,9 +614,9 @@ public sealed class BSCharacter : BSPhysObject
     public override OMV.Vector3 PIDTarget {
         set { _PIDTarget = value; }
     }
-    public override bool PIDActive {
-        set { _usePID = value; }
-    }
+
+    public override bool PIDActive { get; set; }
+
     public override float PIDTau {
         set { _PIDTau = value; }
     }