Add equal-strength drawrate to log output

[kiudee]

We recently added an estimate of the drawrate of the current match. This is useful to determine if the book used for tuning is suitable.
The specific drawrate is of course dependent on the Elo difference between both playing engines.
To get a more invariant measure of the draw rate of the book, we should use the "equal strength drawrate", that is the drawrate which would result if both engines were actually of equal strength.

It is possible to compute this using:

d_equal = d / (2 * sqrt(-w * (d + w - 1)) + d)