Add miscellaneous undead-related fixes. When changing species to

vampire in wizard mode, set you.is_undead to the proper value; check for you.is_undead's being US_UNDEAD when determining if a species can eat, instead of checking for the mummy species directly; and display the "if you weren't so full, you could turn into a bat" message for vampires that gain the ability when at least full, instead of only when satiated, since it won't work in the former case.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4476 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on April 22, 2008
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