numfmt: support lowercase 'k' for Kilo and Kibi

numfmt: support lowercase 'k' for Kilo and Kibi

For consistency with the "SI" standard, and with other coreutils
which output a lowercase 'k' in "SI" mode.

* src/numfmt.c (suffix_power): Treat 'k' like 'K' on input.
(double_to_human): Output lowercase 'k' in SI mode.
(usage): Adjust accordingly.
* doc/coreutils.texi: Mention 'k' accepted, and printed in SI mode.
* tests/misc/numfmt.pl: Adjust accordingly.
* NEWS: Mention the change in behavior.
Fixes https://bugs.gnu.org/47103