Add support for auto-generating info/dir

Add support for auto-generating info/dir

* admin/update_autogen: Add option to generate info/dir.
(Usage): Add -I.
(info_flag): New variable.
(-I): New option.
(doc): Maybe check its status.
(info_dir): New function.

* admin/dir_top: New file.

* doc/emacs/emacs.texi:
* doc/lispintro/emacs-lisp-intro.texi:
* doc/lispref/elisp.texi:
* doc/misc/autotype.texi, doc/misc/cc-mode.texi, doc/misc/ediff.texi:
* doc/misc/ert.texi, doc/misc/htmlfontify.texi, doc/misc/ido.texi:
* doc/misc/octave-mode.texi, doc/misc/org.texi, doc/misc/srecode.texi:
* doc/misc/todo-mode.texi, doc/misc/tramp.texi:
Sync direntry with info/dir version.