Switch back to non-IEC units for 1024 bytes

I was swayed a little too quickly when I saw the wiki page discussing
kB vs KiB. Switch back as none of the code in base openly uses
IEC units via humanize_number(3) (which was my next step), and there's
a large degree of dislike with IEC vs more SI-like units.

MFC after: 7 weeks
Submitted by: jhb, rgrimes
Sponsored by: Dell EMC Isilon