Move dired-info and dired-man from dired-x to dired

Move dired-info and dired-man from dired-x to dired

* lisp/dired-x.el (dired-bind-man, dired-bind-info): Change into
defvars and make obsolete.
(dired-extra-startup): Doc fix.
(dired-info, dired-man): Move from here...
* lisp/dired.el (dired-do-info, dired-do-man): ...to here and
rename.  (Bug#21981)

* lisp/dired.el (dired-mode-map): Bind 'I' and 'N' to 'dired-do-info'
and 'dired-do-man'.

* doc/misc/dired-x.texi (Miscellaneous Commands): Remove documentation
of 'dired-info' and 'dired-man'.
* doc/emacs/dired.texi (Dired Enter): Document 'dired-do-info' and
'dired-do-man'.
* etc/NEWS: Announce the above changes.