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.
Fix dbg_mean_of() for negative numbers

Type should be int64_t instead of uint64_t

No functional change.
Rename castling flag to castling right

This is a more conventional naming as
reported also in:

http://chessprogramming.wikispaces.com/Castling+rights

No functional change.