"The percentage value shall be expressed as a positive integer, with any
fractional result causing it to be rounded [up] to the next highest
integer."
-- https://pubs.opengroup.org/onlinepubs/9799919799/utilities/df.html
Use a hand-rolled ceil() equivalent (as long as it doesn't overflow), to
avoid using math.h. Use an unsigned decimal number to express the
percentage. Keep the current width of the field to avoid changing the
spacing between columns.
Fix the tests accordingly, they now match the percentage values on
NetBSD.
Also, use the same rounding method for the calculation of used inodes.