Be more optimistic in aspiration window

Be more optimistic wrt search instability, and set the unviolated bound
half window.

STC
LLR: 2.96 (-2.94,2.94) [-1.00,4.00]
Total: 16362 W: 3371 L: 3197 D: 9794

LTC
LLR: 2.94 (-2.94,2.94) [0.00,5.00]
Total: 21666 W: 3780 L: 3569 D: 14317

Bench: 6807896

Resolves #105
Codestyle massage Search::init()

* remove some erroneous comments, that were based on the ONE_PLY == 2.
* rename hd to d, because there's no more half-depth in SF.
* rescope variables into the for loops.
* reindent the for loops correctly.
* add a comment to explain the eval improving part (not so obvious to read
this code as array has 4 dimensions).

No functional change.
Null search extension

If in a PvNode null search fails low while a
reduced search fails high then it means there
is some tension in the position and it makes
sense to extend the search.

This patch just introduces the infrastructure
without enabling the extension.

No functional change.
Enable null-search extension

bench: 6961292
Set threshold at 100 below alpha

bench: 7026128