Add a power parameter to monsters::drain_exp(), and use it for the mass

drain card effect.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8972 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on February 7, 2009
CZWERNJVMJFBGKQHHR6I5U2FCRUJD7H2DG3C43V2CBAW74GYU2HAC
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