Another cleanup of dungeon features. Adding general markers

for first branch/last branch and first altar/last altar that should hopefully make adding new ones easier.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2324 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on October 4, 2007
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