ox-texinfo:: Require only TEXINFO_DIR_CATEGORY

ox-texinfo:: Require only TEXINFO_DIR_CATEGORY

Until now @dircategory/@direntry entries were added only if
both TEXINFO_DIR_CATEGORY and TEXINFO_DIR_TITLE were set.
And the setting of TEXINFO_DIR_TITLE had to be careful to
provide exactly the right syntax.

This patch changes various things in this regard:
- Only require TEXINFO_DIR_CATEGORY in order to generate
  `@dircategory` and `@direntry`.
- Use the document title by default if TEXINFO_DIR_DESC is missing.
- Use the filename by default when TEXINFO_DIR_TITLE is missing.
- Try and make it harder to provide a direntry that does not
  have the right format or refers to a different filename than
  the one we're outputting to.

* lisp/org/ox-texinfo.el: Remove redundant `:group` arguments.
Prefer #' to quote function names.
(org-texinfo-template): Use sane defaults for `@direntry`.

* doc/misc/org.org (Texinfo specific export settings): Adjust accordingly.