Let promotion zone be 3 ranks on 8-rank shogi boards

Let promotion zone be 3 ranks on 8-rank shogi boards

For the benefit of Euro-Shogi the rule that the depth of the promotion
zone is the board height divided by 3, rounded down (which works so well
for mini-, Judkins, Tori and regular Shogi) is given an exception when
the numer of ranks is 8.