[Cfp-interest 1925] action item: 754-2019 setpayload and setpayloadsig vs. sign bit

[David Hough CFP]

The payload of a NaN is defined in 6.2.1 for binary interchange formats and 3.5
for decimal interchange formats.    It's the T field in the significand.
The sign of the NaN is not part of the payload.

In 9.7, setpayload and setpayloadsig are defined to produce a NaN with
a specified payload.    The sign bit of that NaN is not specified, as with
most operations that produce NaN results from numeric operands.