Rename xxx_to_char() -> to_char()

No functional change.
Remove limit of minimumSplitDepth

There is no reason why an user cannot set
it at a value less than 4.

No functional change.
Pass CastlingFlag argument only

Instead of Color and CastlingSide. Change functions API
accordingly.

No functional change.
Revert dynamic draw value

When tested with weaker engines did not
performed as expected, actually it was even
a regression from standard version.

bench: 8430785
Some minor cleanup stuff

I came across while browsing the code.

No functional change.
Merge pull request #3 from mcostalba/master

sync with master
Merge branch 'master' into tweak-probcut-precondition