Reenable level-leaving monsters' using shaft traps again, properly this

time, and clean up a few related bits. Note that such monsters don't properly disappear yet after falling through shafts.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5946 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on June 18, 2008
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