When converting to SI or IEC, produce values that align with the conventions
used by GNU numfmt.
- values > 10 are represented without a decimal place, so 10000 becomes 10K
instead of 10.0K
- when truncating, take the ceiling of the value, so 100001 becomes 101K
- values < 10 are truncated to the highest tenth, so 1001 becomes 1.1K
closes#1726
