Add infrastructure for keeping track of board position

Add infrastructure for keeping track of board position

A Xiangqi board is added, with code to set it up from a FEN, convert it
to the board part of a FEN, and apply the moves from the move list to it,
keeping track of what was the last capture. The FEN for setting it up
is always taken from the "fen <FEN>" position command ('iniPos') of the
Cyclone dialect used in variant xiangqi.