Ai wander improvements

Small change to allow interrupting walking NPC's.  Fair bit of changes to pathfinding - there were several issues there, the main one being the generated path including unreachable pathpoints.  It appears that vanilla had content not properly cleaned - attached is a pic of a set of unconnected pathgrid points near x=46903, y=-90109.
![orphaned-pathgrid-points](https://cloud.githubusercontent.com/assets/6752530/2557918/e81bdf02-b723-11e3-9882-1ac2c407c1ad.jpg)