Implement variant seirawan

Implement variant seirawan

Set upthe position, including Hawk and Elephant in holdings, in
InitPostion(). Suppress putting captured pieces in holding. Let
promoChar indicate gating in ApplyMove(). Remove gated pieces from
the holdings there. Let GenLegal() generate RxK-type castlings to act as
base for gating on Rook square. Gatings are tested for legality
based on the holdings content and whether they occur on the back
rank. Disambiguate() also tests this. Make sure the gated pieces are
printed as move suffixes in CoordsToAlgebraic().