Add new user option to Gnus to allow `#' to toggle

Add new user option to Gnus to allow `#' to toggle

* doc/misc/gnus.texi (Marking Groups, Setting Process Marks):
Mention the new variable.

* lisp/gnus/gnus-group.el (gnus-group-make-menu-bar): Update menu.
(gnus-group-mark-group): Support the variable.
(gnus-group-mark-update): New command.
(gnus-group-unmark-group, gnus-group-mark-region): Pass in new
parameter.

* lisp/gnus/gnus-sum.el (gnus-summary-make-menu-bar): Update menu.
(gnus-summary-mark-as-processable): Use the variable.

* lisp/gnus/gnus-topic.el (gnus-topic-mark-topic):  (bug#48683).

* lisp/gnus/gnus-topic.el (gnus-topic-mark-topic): Use the variable.

* lisp/gnus/gnus.el (gnus-process-mark-toggle): New user option.