bug#6995: 24.0.50; get-free-disk-space doesn't return available space in dired

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Date: Tue, 07 Sep 2010 16:36:01 -0400
> Cc: 6995@debbugs.gnu.org
> 
>    df -Pk /home/user
> 
> returns
> 
>   Filesystem         1024-blocks      Used Available Capacity Mounted on
>   /real_directory/home/user 461097104 333392304 127704800      73% /home/user
> 
> Because the filesystem name is long, the numerical values do not line
> up with the column headings. The current code seems to assume that
> they do line up. Thus, running dired on "~" gets wrong values for the
> total size. 

How about dropping the use of `df', at least on GNU/Linux?  It's IMO
ridiculous to invoke an external program for a single system call.  I
know that this system call is notoriously non-portable, and that doing
in Emacs all that system-dependent stuff that `df' does is not a good
idea.  But at least on the most popular free system (and perhaps on a
few compatible ones) we could do it right without all that labor, and
gain stability and reliability that depending on `df' will never
achieve.