bug#25942: 26.0.50; dired-mark-extension prepend '.' to suffix if not present

[Juri Linkov]

> emacs -Q /tmp
>
> (require 'dired-x)
> (require 'ert)
> ;; Evaluate following sexp:
> (let ((dirs '("Public" "Music"))
>       (files '(".bashrc" "bar.c" "foo.c"))
>       (dir (make-temp-file "Bug25942" 'dir)))
>   (unwind-protect
>       (progn
>         (dolist (d dirs)
>           (make-directory (expand-file-name d dir)))
>         (dolist (f files)
>           (write-region nil nil (expand-file-name f dir)))
>         (dired dir)
>         (dired-mark-extension "c")
>         (should (= 2 (length (dired-get-marked-files)))))
>     (delete-directory dir 'recursive)))
>
> IMO, in this example looks better if just those files ending with
> '.c' are marked i.e., to not mark 'Public' or 'Music'.

Is this a backward-compatible change?  If not, then maybe better
prepend ‘.’ only to the default value for interactive uses?  So users
will get the right result when using ‘M-x dired-mark-extension RET RET’.
But for the programmatic uses it's the responsibility of the caller to
supply the right suffix in ‘(dired-mark-extension ".c")’