Introduce concept of double pawn protection.

Exclude doubly protected by pawns squares when calculating attackers on
king ring. Idea of this patch is not to count attackers if they attack
only squares that are protected by two pawns.

STC
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 70040 W: 15476 L: 15002 D: 39562
http://tests.stockfishchess.org/tests/view/5c0354860ebc5902bcee1106

LTC
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 16530 W: 2795 L: 2607 D: 11128
http://tests.stockfishchess.org/tests/view/5c0385080ebc5902bcee14b5

This is third king safety patch in recent times so we probably need
retuning of king safety parameters.

Bench: 3057978

src/bitboard.h

@@ -177,6 +177,16 @@ constexpr Bitboard pawn_attacks_bb(Bitboard b) {
 }
 
 
+/// double_pawn_attacks_bb() returns the pawn attacks for the given color
+/// from the squares in the given bitboard.
+
+template<Color C>
+constexpr Bitboard double_pawn_attacks_bb(Bitboard b) {
+  return C == WHITE ? shift<NORTH_WEST>(b) & shift<NORTH_EAST>(b)
+                    : shift<SOUTH_WEST>(b) & shift<SOUTH_EAST>(b);
+}
+
+
 /// adjacent_files_bb() returns a bitboard representing all the squares on the
 /// adjacent files of the given one.
 

src/evaluate.cpp

@@ -309,7 +309,7 @@ namespace {
         attackedBy[Us][Pt] |= b;
         attackedBy[Us][ALL_PIECES] |= b;
 
-        if (b & kingRing[Them])
+        if (b & kingRing[Them] & ~double_pawn_attacks_bb<Them>(pos.pieces(Them, PAWN)))
         {
             kingAttackersCount[Us]++;
             kingAttackersWeight[Us] += KingAttackWeights[Pt];