Allow using floor="other_feature" when initialising a one-way stair, so that the stair is replaced with other_feature instead of regular floor (bobbens).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2724 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on November 2, 2007
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