bug#47103: numfmt: invalid suffix 'k'

[Sven Köhler]

So Pádraig's patch does allow for parsing lowercase k, but it does not 
change numfmt to use lowercase k in its output in si mode.

As Pádraig has shown, ls uses lowercase k in --si mode. So it uses 
lowercase k for 1000. I think that numfmt should behave the same for 
consistency reasons.

Also, the texinfo then seems incomplete. It lists kB but not k alone, as 
used by ls in --si mode.