Better perft integration in benchmark

Now with:

   stockfish bench 128 1 5 default perft

it is possible to get perft 5 results of each position and
the first 3 positions correspond to the well known test
position in:

http://chessprogramming.wikispaces.com/Perft+Results

This allow to quickly check for perft consistency running
the 'bench' command.

No functional change but signature has changed because
bench default positions 2 and 3 have changed.

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