Adjust initiative in pure pawn endgames

Adjust initiative in pure pawn endgames

King and pawn endgames are typically decisive, and a small
advantage is often sufficient to win. Therefore we now take
this into account when computing the initiative adjustment.

This idea came from a series of patches by Gian-Carlo Pascutto.

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 48770 W: 10203 L: 9845 D: 28722
http://tests.stockfishchess.org/tests/view/5aa58cce0ebc59029780ff8d

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 22252 W: 3572 L: 3366 D: 15314
http://tests.stockfishchess.org/tests/view/5aa5b27c0ebc59029780ffad

Ideas for future developement:

- There have been a number of changes to the initiative
  calculation lately. Perhaps the coefficients could be
  tuned again.

- It may be possible to add special knowledge for other
  endgames in the initiative calculation.

Closes https://github.com/official-stockfish/Stockfish/pull/1481

Bench: 5750110