Add stochastic torment resistance per dpeg

The mutation comes in three levels and blocks 20%, 40%, or 60% (shown to the user) of torments. Right now, you can cast Symbol of Torment with it; I doubt this is too good. It is on the tier-1 list for demonspawn mutations, and I expect it to be balanced appropriately.

Signed-off-by: Darshan Shaligram <dshaligram@users.sourceforge.net>

Created by  Stefan O'Rear  on October 26, 2009
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