sfio: correct floating decimal point scaling of fractions (#131)

_sfcvt(), "convert a floating point value to ASCII", did not adjust
for negative decimal place movement as what happens with leading
zeroes. This caused ksh's 'printf %f' formatter to fail to round
floating point values correctly.

src/lib/libast/sfio/sfcvt.c:
- Removed constraint of <1e-8 for doubles by matching what was done
  for long doubles having <.1.
- Corrected a condition when the next power of 10 occurred and that
  new 1 digit was being overwritten by a 0.

 src/cmd/ksh93/tests/math.sh:
- Validate that typeset -E/F formatting matches that of their
  equivalent printf formatting options as well as checking for
  correct float scaling of the fractional parts.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-14:
+
+- Corrected rounding of floating point values by ksh's printf %f formatting
+  operator. Fix contributed by @hyenias.
+
 2020-09-11:
 
 - The 'command' regular builtin utility (which runs a simple command, removing