Use new Gnulib function dtoastr() to format short, accurate real numbers.

Use new Gnulib function dtoastr() to format short, accurate real numbers.

%.*g with DBL_DIG + 1 as argument is simple but in rare cases it fails to
accurately format a real number.  The recently added Gnulib routine
dtoastr() always formats a real number accurately, so switch to using it
for these cases.