Move dired-jump from dired-x to dired (Bug#21981)

Move dired-jump from dired-x to dired (Bug#21981)

* lisp/dired-x.el (dired-bind-jump): Change into defvar and make
obsolete.
(dired-extra-startup): Doc fix.
(dired-jump, dired-jump-other-window): Move from here...
* lisp/dired.el (dired-jump, dired-jump-other-window): ...to here.

* lisp/bindings.el (ctl-x-map, ctl-x-4-map): Bind 'C-j' to
'dired-jump' and 'dired-jump-other-window'.

* doc/misc/dired-x.texi (Features, Installation)
(Miscellaneous Commands): Remove documentation of 'dired-jump'.
* doc/emacs/dired.texi (Dired Enter): Document 'dired-jump' and
dired-jump-other-window.
* etc/NEWS: Announce the above changes.

* test/lisp/dired-tests.el (dired-autoload): Adjust test.