* texinfo.tex (\heading, \subheading, \subsubheading): Can no
longer call the nonexistent \*secheadingi series. Instead, call
\plain*secheading.
(\plainsubsecheading, \plainsubsubsecheading): New macros, by analogy
with \plainsecheading.
(\unnumberedsubseczzz, \unnumberedsubsubseczzz): Call them.
(\smalllispx): Remove \setleading{10pt}. That was too small.
(\doprintindex): Do not call \tex ... \Etex. Index files are Texinfo
source, not TeX source, except for using \ instead of @ as the
escape character (for now).
