Change how Dired displays available space

Change how Dired displays available space

* doc/emacs/dired.texi (Misc Dired Features): Document it (bug#23812).
* lisp/dired.el (dired-free-space): New user option.
(dired-insert-directory): Use it from here.
(dired--insert-disk-space): New function that uses the user option.

* lisp/files.el (insert-directory): Don't transform "total" here.
* lisp/ls-lisp.el (ls-lisp--insert-directory): Or here.  Instead
just leave the "total <num>" bit alone, and let Dired transform it.

* test/lisp/files-tests.el (files-tests): Move "available" tests
to dired-tests.
* test/lisp/dired-tests.el (data-dir): Moved here.