[1748837] Merge identical features when reporting explore discoveries,

distinguish between stairs and portals.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@1770 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dshaligram  on July 6, 2007
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