Function: get-free-disk-space

Return the amount of free space on directory DIR's file system.
The return value is a string describing the amount of free
space (normally, the number of free 1KB blocks).

This function calls `file-system-info' if it is available, or
invokes the program specified by `directory-free-space-program'
and `directory-free-space-args'. If the system call or program
is unsuccessful, or if DIR is a remote directory, this function returns nil.