Revert the elemental staff special damage to 3d((1+skill+evo)/2) again. Fix a bug that prevented the special damage costing 2 mp per kerpoww. Staff of earth keeps taking monster AC to account.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@10374 c06c8d41-db1a-0410-9941-cceddc491573

Created by  evktalo  on July 22, 2009
IRED6GUO7CWJFBZOQWQQPBUIFOUU35A5SUM5EWOIN2A4CZD5H3SAC
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