Add a few more corpse and zombie-related cleanups. Swamp worms now

leave zombifiable contaminated corpses, for consistency with the other worms. Also, quicksilver and iron dragons now have weight, so they now leave corpses in addition to their already being zombifiable.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8204 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on January 4, 2009
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