Improve multiPV mode

Skip all moves during the Non-PV (zero-window) search which will be
searched as PV moves later anyways. We also wake sure the moves will
be reported to the GUI despite they're not being searched — some GUIs
may get confused otherwise, and it would unnecessarily complicate the
code.

Tested with MultiPV=4
STC
http://tests.stockfishchess.org/tests/view/5ce7137c0ebc5925cf070d69
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 8233 W: 3708 L: 3424 D: 1101

LTC
http://tests.stockfishchess.org/tests/view/5ce798d60ebc5925cf071d17
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 7369 W: 3197 L: 2911 D: 1261

Closes https://github.com/official-stockfish/Stockfish/pull/2163

No functional change. (in single PV mode)

src/thread.h

@@ -59,7 +59,7 @@ public:
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;
-  size_t pvIdx, pvLast, shuffleExts;
+  size_t pvIdx, multiPV, pvLast, shuffleExts;
   int selDepth, nmpMinPly;
   Color nmpColor;
   std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;