bug#28639: 26.0.60; get-free-disk-space gives wrong number on OS X 10.8 and later

[YAMAMOTO Mitsuharu]

The function `get-free-disk-space' in files.el gives free inode count
rather than free data block space on OS X 10.8 and later.  This is
because it relies on the "df" command and parse the output as follows:

            ;; Assume that the "available" column is before the
            ;; "capacity" column.  Find the "%" and scan backward.
            (goto-char (point-min))
            (forward-line 1)
            (when (re-search-forward
                   "[[:space:]]+[^[:space:]]+%[^%]*$"
                   (line-end-position) t)
              (goto-char (match-beginning 0))
              (let ((endpt (point)))
                (skip-chars-backward "^[:space:]")
                (buffer-substring-no-properties (point) endpt)))))))))

The output on OS X 10.8 looks like this:

Filesystem    512-blocks      Used  Available Capacity iused      ifree %iused  
Mounted on
/dev/disk2    1996082176 172984304 1822585872     9% 2508930 4292458349    0%   
/
devfs                371       371          0   100%     642          0  100%   
/dev
map -hosts             0         0          0   100%       0          0  100%   
/net
map auto_home          0         0          0   100%       0          0  100%   
/home

So, `get-free-disk-space' adopts the "ifree" column rather than the
"Available" column.

One could use the "-P" option to suppress the inode columns.  But this
doesn't seem to be smart (cf. Bug#6995).

Some platforms rely on the primitive function `file-system-info'
instead of the "df" output to get free space.  How about implementing
`file-system-info' also for other platforms using the fsusage module
in Gnulib?

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

In GNU Emacs 26.0.60 (build 1, x86_64-apple-darwin16.7.0, X toolkit, Xaw3d 
scroll bars)
 of 2017-09-29 built on YAMAMOTO-no-iMac-5K.local
Repository revision: af130f900fc499f71ea22f10ba055a75ce35ed4e
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
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