dired-mark-extension: Unmark if called with C-u prefix

dired-mark-extension: Unmark if called with C-u prefix

See discussion in #Bug2518 and:
https://lists.gnu.org/archive/html/emacs-devel/2016-09/msg00711.html
* lisp/dired-x.el (dired-mark-extension):
Update interactive calls: a prefix arg C-u unmark files;
a prefix C-u C-u prompt for MARKER-CHAR and mark files with it.
(dired-mark-sexp):
Show in the prompt that we are unmarking if called with a prefix argument.
* doc/misc/dired-x.texi (Advanced Mark Commands):
Update documentation for 'dired-mark-extension'.