Shallow search to verify probcut

Shallow search to verify probcut

Perform a preliminary shallow search to verify a probcut before doing
the normal "depth - 4 plies" search.

STC:
LLR: 4.73 (-2.94,2.94) [0.00,5.00]
Total: 36281 W: 8221 L: 7830 D: 20230
http://tests.stockfishchess.org/tests/view/5a921cb90ebc590297cc87f6

LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 22907 W: 3954 L: 3738 D: 15215
http://tests.stockfishchess.org/tests/view/5a92672b0ebc590297cc8814

Happy to see something from Ethereal work for Stockfish :)

Bench: 5882274