Add file sorting options to find-dired and grep-find (bug#36110)

Add file sorting options to find-dired and grep-find (bug#36110)

* lisp/find-dired.el (find-ls-option-default-ls)
(find-ls-option-default-exec, find-ls-option-default-xargs):
New variables for values used for options of 'find-ls-option'.
(find-ls-option): Use these variables for default values and options.
(find-dired-refine-function): Refine :type.

* lisp/progmodes/grep.el (grep-find-use-xargs): Use defcustom
instead of defvar.  Add new value 'gnu-sort'.
(grep-compute-defaults): Handle new 'gnu-sort' option of
'grep-find-use-xargs'.