Implement passive mapping

As you explore, a halo of mapping tracks you. The halo includes a percentage of tiles within a distance of every seen tile; optimal play dictates seeing every tile, which just happens to be what autoexplore does. Added as random generation and to the demonspawn tier-2 list.

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

Created by  Stefan O'Rear  on October 26, 2009
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