Added King of the Hill variant support (based on SCIDB 3-checks variant code)

I'm looking for a good way to test this change (whereby moving a king to the center wins).

So far I have tested from these positions (and similar positions):
position startpos moves e2e3 e7e6 e1e2 e8e7 e2d3 e7d6 and Stockfish finds 4. Kd4#
position fen k7/p7/8/8/8/8/8/7K w - - 0 1 and Stockfish finds 1. Kg2 a5 2. Kf3 a4 3. Ke4#