sj: Move df(1) into the new free_space() function

5b03735a-038b-11e4-84be-c80aa9e67bbd
sj: Add --allspace option

2f2b79a6-03ab-11e4-a82d-c80aa9e67bbd
2d0b7d50-03ac-11e4-ab70-c80aa9e67bbd
sj: Move "space" section to make it alphabetical

7f8e3794-03ab-11e4-ab23-c80aa9e67bbd