Fix 1895579: Uniques dropping their specific artefacts.

Asmodeus now won't drop the Sceptre, Dispater not the Staff, and Cerebov will also keep his Sword.

Also, monsters will only pick up potions if they can use them, i.e. only vampires drink blood, and undead and non-living can't use !healing.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@4254 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on April 16, 2008
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