isinf() on NaN args

[Bruce Evans]

Is isinf(x) (or even fpclassify(x)) permitted to raise FE_INVALID if
x is a (quiet) NaN?  It shouldn't, but I can't find where C99 requires
this.  I can only find where C99 requires FE_INVALID to not be raised
for comparison macros.

It should raise if x is a signaling NaN, but C99 doesn't specify
signaling NaNs and the hardware behaviour is more varied and the
software bugs are larger for signaling NaNs, so getting this wrong in
many cases doesn't matter much.

Bruce