Score unopposed weak pawns only if majors

Do not use the opposed flag for scoring backward and isolated pawns
in pawns.cpp, instead give a S(5,25) bonus for each opponent unopposed
weak pawns when we have a rook or a queen on the board.

STC run stopped after 113188 games:
LLR: 1.63 (-2.94,2.94) [0.00,5.00]
Total: 113188 W: 20804 L: 20251 D: 72133
http://tests.stockfishchess.org/tests/view/59b58e4d0ebc5916ff64b12e

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 66673 W: 8672 L: 8341 D: 49660
http://tests.stockfishchess.org/tests/view/59b902580ebc5916ff64b231

This is Alain Savard's idea, just with a different bonus.
Original patch there:

green STC, http://tests.stockfishchess.org/tests/view/597dcd2b0ebc5916ff64a09b
yellow LTC, http://tests.stockfishchess.org/tests/view/597ea69e0ebc5916ff64a0e6

Bench: 6259498

src/evaluate.cpp

@@ -162,8 +162,8 @@ namespace {
   // supported by a pawn. If the minor piece occupies an outpost square
   // then score is doubled.
   const Score Outpost[][2] = {
-    { S(22, 6), S(33, 9) }, // Knight
-    { S( 9, 2), S(14, 4) }  // Bishop
+    { S(22, 6), S(36,12) }, // Knight
+    { S( 9, 2), S(15, 5) }  // Bishop
   };
 
   // RookOnFile[semiopen/open] contains bonuses for each rook when there is no
@@ -214,6 +214,7 @@ namespace {
   const Score ThreatBySafePawn    = S(182,175);
   const Score ThreatByRank        = S( 16,  3);
   const Score Hanging             = S( 48, 27);
+  const Score WeakUnopposedPawn   = S(  5, 25);
   const Score ThreatByPawnPush    = S( 38, 22);
   const Score HinderPassedPawn    = S(  7,  0);
   const Score TrappedBishopA1H1   = S( 50, 50);
@@ -593,6 +594,10 @@ namespace {
             score += ThreatByKing[more_than_one(b)];
     }
 
+    // Bonus for opponent unopposed weak pawns
+    if (pos.pieces(Us, ROOK, QUEEN))
+        score += WeakUnopposedPawn * pe->weak_unopposed(Them);
+
     // Find squares where our pawns can push on the next move
     b  = shift<Up>(pos.pieces(Us, PAWN)) & ~pos.pieces();
     b |= shift<Up>(b & TRank3BB) & ~pos.pieces();