Phobos request: function to get the available disk space

[Johan Engelen via Digitalmars-d]

Hope I can motivated someone to work on adding a function to 
Phobos that returns the available disk space (on a given path): 
https://issues.dlang.org/show_bug.cgi?id=16487

(I needed this for file cache management in LDC, and was 
surprised I had to write my own.)

I think it'd be a valuable addition, especially because it is 
non-trivial to make cross-platform and hard to test whether it 
actually works.

Much obliged,
  Johan