After some pointers from jpeg, rework the routine for monsters' leaving

the level to use the pathfinding routines for patrolling. The monsters still sometimes disappear when near the exit instead of on it, which needs to be fixed, but it's vastly closer to working properly.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6186 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on June 28, 2008
SPWOUV6ZNHLBSAX455ACJDBEJYVFHGO4H76L4NPAE5DF6BEAK34QC
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