Symmetric King Safety: take 2

Another attempt at retiring current asymmetric
king evaluation and use a much simpler symmetric
one. As a good side effect we can avoid recalculating
eval after a null move.

Tested in no-regression mode and passed

STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 21580 W: 3752 L: 3632 D: 14196

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 18253 W: 2593 L: 2469 D: 13191

And a LTC regression test against SF DD to
verify we don't have regression against
weaker engines due to some kind of 'contempt'
effect:

ELO: 54.69 +-2.1 (95%) LOS: 100.0%
Total: 40000 W: 11072 L: 4827 D: 24101

bench: 8205159

src/search.h

@@ -45,7 +45,8 @@ struct Stack {
   Move killers[2];
   Depth reduction;
   Value staticEval;
-  int skipNullMove;
+  bool skipNullMove;
+  bool nullChild;
 };
 
 
@@ -101,7 +102,6 @@ extern volatile SignalsType Signals;
 extern LimitsType Limits;
 extern std::vector<RootMove> RootMoves;
 extern Position RootPos;
-extern Color RootColor;
 extern Time::point SearchTime;
 extern StateStackPtr SetupStates;