Add a OS_NPC_LAND_AT_TARGET option to osMoveToTarget()

Default for this function is now not to automatically land.
This allows better control by scripts when an avatar is going to be landing on a prim rather than the ground.
Stopping the avatar involves faking a collision, to avoid the pid controller making it overshoot.
A better approach would be to gradually slow the avatar as we near the target

OpenSim/Region/Physics/OdePlugin/ODECharacter.cs

@@ -305,10 +305,12 @@ namespace OpenSim.Region.Physics.OdePlugin
                 {
                     m_iscolliding = true;
                 }
+
                 if (m_wascolliding != m_iscolliding)
                 {
                     //base.SendCollisionUpdate(new CollisionEventUpdate());
                 }
+
                 m_wascolliding = m_iscolliding;
             }
         }