- fixed __checkSystemDiskData runtime check to work correctly
  with the 'all' size specification
- fixed and optimized disk size calculation
  * I had by default 40% spare space on the root path which seems to
    be too much, I have 20% spare on each volume so I adapted that
    to the root path as well.
  * for the size estimation I now use the --apparent-size option in
    du, which works better on files with holes and other or other
    attributes which makes a difference when copying those data.
  * Use a fixed value for the filesystem overhead each volume adds
    to the disk. It is wrong to calculate and add a 20% spare/volume
    according to the size of the data in the volume to the disk,
    because that value has already been taken into account with
    the data spare calculation of the entire tree