Allow getting the slow healing mutation.

Be less nice when a god gifts a negative mutation: clear away the appropriate positive mutation (e.g., eliminate Regeneration if a god gifts Slow Healing), instead of the previous behaviour, which was to fail. Fix bug where demonspawn with the genetically-stable mutation could get conflicting mutations. Clean up mutation code.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9333 c06c8d41-db1a-0410-9941-cceddc491573

Created by  haranp  on March 5, 2009
4LFTDJY3DK6S7C7SQLLML6UIZ42GZOWK2EIMZZSNEKBWSA3YNUZAC
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