texinfo.el: fails to build menus with @appendix

[Akim Demaille]

texinfo.el is still unable to built menus when there are appendices.
This is very troublesome, since many GNU documents do include fdl.texi
and such, which are precisely @appendixsec.

Just try a simple C-c C-u C-a on the attached file, and see the
result:

appendix-menus-are-improperly-updated.texi
Description: TeXInfo document

                     | \input texinfo @c -*-texinfo-*-
                     | @c %**start of header
                     | @setfilename appendix.info
                     | 
                     | @ifnottex
                     | @node Top
                     | @top Appendix
                     | @end ifnottex
                     | 
                     | @menu
                     | * Introduction::
                     | * Appendices::
This is wrong: it -->| * Index::
is a subsection, not | @end menu
a chapter.           | 
                     | @node Introduction
                     | @chapter Introduction
                     | 
                     | @node Appendices
This is (i) empty,   | @menu
(ii) between @node   | @end menu
and @appendix!!!     | 
                     | @appendix Appendices
                     | 
                     | @node Index
                     | @appendixsec Index

Thanks!

(This is Emacs 21.3.1's Texinfo.el.)