Fix startpos_ply_counter() regression

Return the correct number of played plies at the end
of the setup moves. Currently it always returns 0 when
starting from start position, like in real games.

We fix this adding st->pliesFromNull that starts from 0
and is incremented after each setup move and is never
reset in the setup phase but only once search is started.

It is an hack because startpos_ply_counter() will return
different values during the search and is correct only
at the beginning of the search.

No functional change.

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