New sewer entry vault; also altered sewer_kobolds a bit - there seemed to be a place that you could get stuck in.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@10118 c06c8d41-db1a-0410-9941-cceddc491573

Created by  evktalo  on July 7, 2009
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