fix distcheck, make valid texinfo that can make pdfs

fix distcheck, make valid texinfo that can make pdfs

* Makefile.am (EXTRA_DIST): Add other .texi files to dist.

* doc/low-level-gl.texi:
* doc/low-level-glu.texi:
* doc/low-level-glx.texi:
* figl/low-level/gl.scm:
* figl/low-level/glu.scm:
* figl/low-level/glx.scm: Regenerate.

* figl/parse.scm (*rules*): Avoid <title> elements; headings can't be
  present in @defun sections.  Don't generate @math blocks, as texi2pdf
  does not interpret them well.  Instead use @r.

* maint/update-low-level-bindings (unique-copyrights): Handle lack of
  copyright (shouldn't happen).
  (write-scm): Make plain-text docstrings again.