Make kingRing always 8 squares

Make kingRing always eight squares, extending the bitboard to the
F file if the king is on the H file, and to the C file if the king
is on the A file. This may deal with cases where Stockfish (like
many other engines) would shift the king around on the back rank
like g1h1, not because there is some imminent threat, but because
it makes king safety look a little better just because the king ring
had a smaller area.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 34000 W: 7167 L: 6877 D: 19956
http://tests.stockfishchess.org/tests/view/5ab8216d0ebc5902932cbe64

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 22574 W: 3576 L: 3370 D: 15628
http://tests.stockfishchess.org/tests/view/5ab84e6a0ebc5902932cbe72

How to continue from there?

This patch probably makes it easier to tune the king safety evaluation,
because the new regularity of the king ring size will make the king
safety function more continuous.

Closes https://github.com/official-stockfish/Stockfish/pull/1512

Bench: 5934103