Use more_than_one() instead of single_bit()

It is more correct given what the function does. In
particular single_bit() returns true also in case of
empty bitboards.

Of course also the usual renaming while there :-)

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/endgame.cpp

@@ -710,7 +710,7 @@ ScaleFactor Endgame<KBPKB>::operator()(const Position& pos) const {
           return SCALE_FACTOR_DRAW;
       else
       {
-          Bitboard path = squares_in_front_of(strongerSide, pawnSq);
+          Bitboard path = forward_bb(strongerSide, pawnSq);
 
           if (path & pos.pieces(KING, weakerSide))
               return SCALE_FACTOR_DRAW;