Improve pacified monsters' routines for leaving the level again. Remove

death by submersion for now, since it's too specific to certain monsters, and leaving the level long enough will make them disappear anyway. Also, have monsters sometimes cycle through previously unreachable areas again, in case things change on the level to make them reachable.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6346 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on July 2, 2008
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