* lisp/dired.el (dired-mark-region): New defcustom (bug#39902)

* lisp/dired.el (dired-mark-region): New defcustom (bug#39902)

* lisp/dired.el (dired-mark-if): Use dired-mark-region.
(dired-mark): Use dired-mark-region.  Fix docstring.
(dired-mark-files-regexp, dired-mark-files-containing-regexp)
(dired-mark-symlinks, dired-mark-directories)
(dired-mark-executables, dired-flag-auto-save-files)
(dired-flag-backup-files): Mention dired-mark-region in docstring.

* lisp/dired-aux.el (dired-compare-directories):
* lisp/dired-x.el (dired-mark-unmarked-files, dired-mark-sexp):
Mention dired-mark-region in docstring.