LibC: `%g` printf format specifier precision should be treated as significant digits

[spholz]

This test code

fprintf(stderr, "%.3g\n", 12.34567);
fprintf(stderr, "%.4g\n", 10.1);

currently prints

12.345
10.0999

However, using glibc, this prints

12.3
10.1

We should treat the precision parameter of %g format specifiers as the number of significant digits, not the number of digits after decimal point.

Additionally, it seems like we are incorrectly rounding 10.1 to 10.0999.

See also POSIX:

Let P equal the precision if non-zero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X:

• If P>X>=-4, the conversion shall be with style f (or F) and precision P-(X+1).
• Otherwise, the conversion shall be with style e (or E) and precision P-1.

For 12.345, style E would be printed as 1.235E+01, so the exponent is X=1.
P=3 is greater than X (and >= -4), so it will be printed with style f and precision P-(X+1)=3-(1+1)=1.