Add the unmount function

Add a function to unmount a file system using either unmount(2) or
umount(8) depending on the underlying operating system.  Just borrow
the already-tested code from the testers module.
Add the free_disk_space function

This new function queries the available free space, in bytes, of a given
file system.
Enforce the required_disk_space requirement

Recognize the new required_disk_space metadata property (provided
as require.diskspace from ATF test programs) and enforce it in the
requirements check.