Use move_pawns() in Pawns::probe

And rename some stuff.

No functional change.
Don't calculate pawnsOnSquares twice

And reformat some code while there.

No functional change.
Rename piece_count and piece_list

No functional change.
Remove redundant condition in probcut

When !ss->skipNullMove it is assured that excludedMove == MOVE_NONE

No functional change.
Merge some if statements in pos_is_ok()

No functional change.

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

Here we skip the call to pos.attacks_from<ROOK>(s) in the 98%
of cases, testing the first 2 members first. Unfortunatly
code is a bit triky and not clear. So we give up to the
speed optimization in exchange of more code clarity.

No functional change.

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