fix for Animation of castling moves in Chess960

fix for Animation of castling moves in Chess960

Animation of castling moves in Chess960 turned out to sometimes make the
king disappear from the display. For the time being I patched this in
xboard.c to suppress animation of FRC castling moves altogether. As FRC
castlings internally are represented as KxR, the to-square does not make
sense anyway, which made the animation look very weird even when it led
not to a king disappearance act. And the Rook move of a castling is never
animated.