Implement King of the Hill as engine-supported variant

Implement King of the Hill as engine-supported variant

The new winning condition is implemented as finding a royal of the stm
on the hill at the start of the turn. A global flag 'hill' enables this
test, and is set when the value of piece #3 is -2. (Kludgy!) The flag
centr[pieceType] is now used as a multiplier, which for 0, 1 amounts
to the same, but opens the possibility to up it before the search, for a
specific piece type. This is used in 'King of the Hill' to increase the
attraction of the King towards the center from 12 to 264cP when material
drops from Q+2minor (each) to nothing. (This was worth 100 Elo!)
The royal piece must be piece #3!