[Cfp-interest] Three round nearest modes and FLT_ROUNDS

[David Hough CFP]

> My understanding is that 754-2019 does not intend for roundTiesToZero to be a new user selectable rounding direction.

Correct, it's a special purpose rounding mode just for the augmented
operations, determined to make the augmented operations as widely useful
as possible.     It's not expected to be useful for other operations.