Fixes Mantis #3260. Thank you kindly, MCortez for a patch that:

llSetHoverHeight() should not clamp the x/y position of an object the way MoveTo does, 
and it should recalculate the absolute height to hover at as an object moves to reflect 
the current ground/water height under it.
Correctly implementing required adjusting the Physics interfaces and implementing at 
the physics plug-in level. The attached is a patch that correctly implements 
llSetHoverHeight() including updates to the ODE physics plug-in.

OpenSim/Region/Physics/BasicPhysicsPlugin/BasicPhysicsPlugin.cs

@@ -447,9 +447,17 @@ namespace OpenSim.Region.Physics.BasicPhysicsPlugin
         public override void CrossingFailure()
         {
         }
+
         public override PhysicsVector PIDTarget { set { return; } }
         public override bool PIDActive { set { return; } }
         public override float PIDTau { set { return; } }
+
+        public override float PIDHoverHeight { set { return; } }
+        public override bool PIDHoverActive { set { return; } }
+        public override PIDHoverType PIDHoverType { set { return; } }
+        public override float PIDHoverTau { set { return; } }
+
+
         public override void SubscribeEvents(int ms)
         {