Clean up the rotting routines a bit.

For the record, rotting attacks from monsters currently have the same problem that draining attacks used to, due to monsters::rot()'s directly modifying the monster's max HP, as well as its directly calling monsters::hurt().

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8340 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on January 8, 2009
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