Implement TSO's halo. It applies backlight to the player and all

monsters within the (circular) halo, and then keeps the duration of that backlight from running out. You currently lose it if you go under penance. Also, there are no visual effects yet, other than the usual backlight effects.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@3669 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on March 15, 2008
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