Display amount of disk space actually available to the user
As explained in bug 429263, the amount of free space on a partition does not match the amount of space that is actually available to the user, which is also shown by Dolphin and the df command. The user may therefore think that there is still space left, while the space there is actually reserved by the filesystem.
This MR changes the amount of space that is shown for a disk from amount free to amount available, which does not include the reserved blocks.