Backend function for file storage space reporting

This adds functionality to retrieve disk space information
for file storage. It calls the 'df' tool and parses its
output to extract the total and free amount of disk space
on the disk where the given path is located.
The code is not integrated yet, but thoroughly unit-tested.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Bernardo dal Seno <bdalseno@google.com>