Enhanced verify search (#1338)

by disabling null-move-pruning for the side to move for first part of
the remaining search tree. This helps to better recognize zugzwang.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 18220 W: 3379 L: 3253 D: 11588
http://tests.stockfishchess.org/tests/view/5a2fa6460ebc590ccbb8bc2f

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 41899 W: 5359 L: 5265 D: 31275
http://tests.stockfishchess.org/tests/view/5a2fcf440ebc590ccbb8bc47

For further detail see commit notes and discussion at 
6401a80ab9

bench: 5776193

src/thread.h

@@ -61,7 +61,7 @@ public:
   Material::Table materialTable;
   Endgames endgames;
   size_t PVIdx;
-  int selDepth;
+  int selDepth, nmp_ply, pair;
   std::atomic<uint64_t> nodes, tbHits;
 
   Position rootPos;