Rename the 'nv' as 'fv' since it's not NV.

To avoid constant confusion.  It actually most of the time explicitly
a "long double".
POSIX math: log1p is more subtle.

-Inf on -1, NaN on < -1.
Separate S_my_atof_infnan().
hexfp: 0 is 0x0p+0.
If USE_LONG_DOUBLE, NVff is PERL_PRIldbl.
Floating point should always output more than nothing.