Increase MAX_PLY from 100 to 256

Increase MAX_PLY from 100 to 256

There is no need to limit the maximum ply searched to
100, with deep exclusion search extensions we could
reach it even with much smaller search depths.

The only drawback is an increase in stack usage, but
is limited mainly to id_loop(), in particular the
recursive search() functions are not affected.

No functional change.

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