numfmt: does not honor LC_NUMERIC for decimal separator (input and output)

[sylvestre]

uutils numfmt always uses . as the decimal separator, regardless of LC_NUMERIC. GNU honors the locale for both formatting output and parsing input.

Output differs

$ LC_NUMERIC=fr_FR.UTF-8 /usr/bin/numfmt --to=iec 1500
1,5K

$ LC_NUMERIC=fr_FR.UTF-8 target/debug/coreutils numfmt --to=iec 1500
1.5K

Same with an explicit format:

$ LC_NUMERIC=fr_FR.UTF-8 /usr/bin/numfmt --to=iec --format=%-17.5f -- 310174
302,90500K

$ LC_NUMERIC=fr_FR.UTF-8 target/debug/coreutils numfmt --to=iec --format=%-17.5f -- 310174
302.90430K

(The digit difference there is tracked separately in #11926 — this issue is strictly about the . vs ,.)

Input parsing differs

GNU switches the input parser to the locale separator as well:

$ LC_NUMERIC=fr_FR.UTF-8 /usr/bin/numfmt --format=%.3f 1.5
numfmt: suffixe incorrect en entrée : '1.5'

$ LC_NUMERIC=fr_FR.UTF-8 /usr/bin/numfmt --format=%.3f 1,5
1,500

uutils accepts 1.5 in any locale and never accepts 1,5:

$ LC_NUMERIC=fr_FR.UTF-8 target/debug/coreutils numfmt --format=%.3f 1.5
1.500
$ LC_NUMERIC=fr_FR.UTF-8 target/debug/coreutils numfmt --format=%.3f 1,5
numfmt: invalid number: '1,5'

Expected

Match GNU: when LC_NUMERIC (or LC_ALL) names a locale whose decimal separator is ,, both the parser and the formatter should use ,.