This patch enables by default the old 'human-readable' option, but in a
slightly different model.

The option is now called "units" and takes either:
 - 'h' for automatic formatting
 - 'm', 'g' or 't' for mebi/gibi/tebibytes

If 'h' is used, we add a unit suffix, otherwise nothing is added so that
parsing is easy.

The default value of this unit is:
  - 'h' if a separator is not passed
  - 'm' if a separator is passed

Reviewed-by: ultrotter