Penalty for Knight when enemy pawns are few

This seems more a material imbalance topic,
anyhow test is good and so patch is applied
as is.

Passed both short TC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 17391 W: 3548 L: 3393 D: 10450

And long TC:
LLR: 3.00 (-2.94,2.94) [0.00,6.00]
Total: 34660 W: 5972 L: 5700 D: 22988

bench: 8291883
Prefer file_bb() to FileBB[]

No functional change.
Retire mirror()

Inline the only caller site.

No functional change.
Sync with 'master'

bench: 8291883
Imbalance: tune bishop pair (take 1)

bench: 8390304
Imbalance: tune bishop pair (take 2)

bench: 8758420
Imbalance: tune bishop pair (take 3)

bench: 8705645
Imbalance: tune bishop pair (take 4)

bench: 8307915
Imbalance: tune pawn (take 1)

bench: 8211361
Imbalance: tune pawn (take 2)

bench: 8827641
Imbalance: tune pawn (take 3)

bench: 9381773
Imbalance: tune pawn (take 4)

bench: 9171356