Move dired-do-relsymlink from dired-x.el to dired.el

Move dired-do-relsymlink from dired-x.el to dired.el

* lisp/dired-x.el (dired-do-relsymlink, dired-make-relative-symlink)
(dired-do-relsymlink-regexp): Move from here...
* lisp/dired-aux.el (dired-do-relsymlink, dired-make-relative-symlink)
(dired-do-relsymlink-regexp): ...to here.  (Bug#21981)

* lisp/dired-x.el: Move keybinding and menu binding from here...
* lisp/dired.el (dired-mode-map, dired-mode-regexp-menu): ...to
here.

* lisp/dired-x.el (dired-keep-marker-relsymlink): Move from here...
* lisp/dired.el (dired-keep-marker-relsymlink): ...to here.  Improve
docstring.

* doc/misc/dired-x.texi (Miscellaneous Commands): Move
documentation of above commands from here...
* doc/emacs/dired.texi (Operating on Files)
(Transforming File Names): ...to here.