df -k fails to select 1024-byte blocks

[jgarzik]

After recent changes (it worked a while ago), df -k fails to select 1024-byte blocks.

cc @fox0

% ./target/release/df | head -2
Filesystem     512-blocks       Used  Available Capacity Mounted on
/dev/disk1s4s1  976490576  152157960  824332616      16% /         

% ./target/release/df -k | head -2
Filesystem     512-blocks       Used  Available Capacity Mounted on
/dev/disk1s4s1  976490576  152157952  824332624      16% /    

[fox0]

#302

[jgarzik]

Fixed by #302