Allow interrupting a walking NPC to trigger a greeting.
Prevent NPC suicides off silt the strider platform in Seyda Neen.  Added some comments as well.  There may be opportunities for some optimization but left that out for now.
My previous analysis of the pathfinding issue was incorrect.  It was in fact caused due to some of the pathgrid points being unreachable.  Instead of returning an empty path in such a scenario, incorrect path + requested destination were being returned.  There was also a defect where past cost was being used for selecting open points.

There is still an unresolved issue where mGraph and mSCComp are being rebuilt unnecessarily.  The check mCell != cell in buildPath() is being triggered frequently. Not sure why.
Minor comment clarification.