Improve compatibility with older versions of the Math-BigRat distribution.
Math-BigInt version 1.999835 works with Math-BigRat version 0.2623 and newer.
Math-BigInt version 1.999836 works with Math-BigRat version 0.2618 and newer.Re-enable upgrading in Math::BigFloat->bdiv().
Fix an error in the enabling/disabling of upgrading/downgrading.
Fix typos and other formatting errors.