Code style and subtle fix in move_is_legal()

Code style and subtle fix in move_is_legal()

A bunch of trivial code style and comment fixes.

Among them there is a real fix for a subtle case
involving promotion moves.

We currently check that a pawn push to 8/1th rank
must be a promotion, but we don't check the contary,
i.e. that a pawn push on a different rank must NOT be
a promotion. Note that, funny enough, we perform this
control for all the other pieces, but not for the pawns!

This patch fixes this really corner case.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>