Halve king eval margin

But still keep the same original
margin for score.

Passed both short TC test
LR: 2.95 (-2.94,2.94)
Total: 3710 W: 845 L: 726 D: 2139

And long TC
LLR: 2.95 (-2.94,2.94)
Total: 57859 W: 10939 L: 10532 D: 36388

bench: 4769737

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Better document what we skip when in check

No functional change.
Rename MoveStack to ExtMove

Stack has no meaning here, while ExtMove (extended move),
better clarifies that we have a move + a score.

No functional change.
Small simplification in space eval scoring

No functional change.
Microptimize gives_check() for castling case

Without patch we have 333198 nps, with patch 334249.

A very small +0.3%, not a lot manily becuase this is a
side path that is taken very few times.

Anyhow idea is correct becuase first 'quick' condition
has an hit rate of about 95%.

No functional change.
Merge branch 'master' of https://github.com/mcostalba/Stockfish