contrib/lisp/org-e-texinfo: Ensure valid export of escaped characters
and remove invalid characters from menu items
* contrib/lisp/org-e-texinfo.el (org-e-texinfo--sanitize-menu): Remove
invalid characters from menu entries and nodes.
(org-e-texinfo--sanitize-contents): Ensure @ { and } are properly
escaped in text and headlines.
(org-e-texinfo--generate-menu-items): Strip invalid characters from
menu titles.
(org-e-texinfo-fixed-width): Escape @ { and } in fixed-width
environments.
(org-e-texinfo-headline): Ensure nodes do not have invalid characters
and escape @ { and } in section titles.
(org-e-texinfo-plain-text): Use proper escape character for @ { and }.