Cleanup KBPsK endgame

* Clarify distinction between strong side pawns and all pawns.
* Simplify and speed-up determination of pawns on the same file.
* Clarify comments.
* more_than_one() is probably faster than pos.count.

Passed STC:
LLR: 2.95 (-2.94,2.94) {-1.50,0.50}
Total: 40696 W: 7856 L: 7740 D: 25100
Ptnml(0-2): 584, 4519, 10054, 4579, 612
http://tests.stockfishchess.org/tests/view/5e6153b1e42a5c3b3ca2e1a9

closes https://github.com/official-stockfish/Stockfish/pull/2574

No functional change.