This allow to retire do_setup_move() and also to simplify
draw detection logic becuase now we always have:
Min(st->rule50, st->gamePly) = st->rule50
This was already true when starting from starting position,
but now is true even when starting from a FEN string because
now we take in account fullmove number in counting gamePly so
that it is always.
st->rule50 <= st->gamePly
No functional change.
Signed-off-by: Marco Costalba <mcostalba@gmail.com>