Introduce pinning bonus

Add a bonus if a slider is pinning an enemy piece.
Idea from Critter.

After 27443 games at 2"+0.05
Mod vs Orig 5900 - 5518 - 16025 ELO +4 (+- 2.7)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Pinner bonus.  Bench: 6021300
More aggressive futilty when coming from null move