Refactor king ring calculation

Refactor king ring calculation

Compute the "double protection by pawns" expression only once
in initialize(), instead of once for each piece in the piece loop.

Passed STC
http://tests.stockfishchess.org/tests/view/5c1506380ebc5902ba11f3b4
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 9494 W: 2191 L: 2045 D: 5258

Inspired by Nick Pelling's test
   http://tests.stockfishchess.org/tests/view/5c144d110ebc5902ba11e4af
and an older test of mine
   http://tests.stockfishchess.org/tests/view/5c0402810ebc5902bcee1fc8

Non functional change.