Simplify other checks (#1337)

Replace an intricate definition with a more natural one.

Master was excluding squares occupied by a pawn which was blocked by a pawn.
This version excludes any squares occupied by a pawn which is blocked by "something"

Passed STC
http://tests.stockfishchess.org/tests/view/5a2f557b0ebc590ccbb8bc0d
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 44211 W: 8009 L: 7928 D: 28274

and LTC
http://tests.stockfishchess.org/tests/view/5a301d440ebc590ccbb8bc80
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 31958 W: 4108 L: 4002 D: 23848

Bench 5000136