Is the default value of directory-free-space-args wrong?

[Zhang Haijun]

The variable is defined as follow:

(defcustom directory-free-space-args
  (purecopy (if (eq system-type 'darwin) "-k" "-Pk"))
  "Options to use when running `directory-free-space-program'."
  :type 'string
  :group ‘dired)

On macOS(10.13.6), with the default value(“-k"), the result of 
get-free-disk-space is the free inodes, which is not expected.
With another value(“-Pk”), the result is the available disk space, which is 
expected.

The manual of df on macOS says:
     -i      Include statistics on the number of free inodes. This option is 
now the default to conform to
             Version 3 of the Single UNIX Specification (``SUSv3'') Use -P to 
suppress this output.

So -P is needed on macOS.