Update German documentation Texinfo and build machinery.

Update German documentation Texinfo and build machinery.

 * cut out obsolete doc/info/de/extract_categories.sh and associated
   files .../de/extract_categories1.sed and .../de/extract_categories1.awk
 * copy doc/info/build_html.sh.in to doc/info/de/build_html.sh.in
   with minor modifications
 * in doc/info/common-lang.mk, generate HTML documentation via build_html.sh
   instead of Maxima private copy of texi2html
 * tell configure.ac to emit doc/info/de/build_html.sh
 * move some Texinfo macros out of doc/info/de/category-macros.texi
   into doc/info/category-macros.texi (the macros in question are for
   references, not categories, but that is where the other reference
   macros are located)
 * move hyphenation rules out of doc/info/de/category-macros.texi
   into doc/info/de/hyphenation.texi
 * delete doc/info/de/category-macros.texi, use doc/info/category-macros.texi
   instead
 * in maxima.texi, declare @documentencoding UTF-8 instead of ISO-8859-1
   since Texinfo files appear to be encoded as UTF-8 (not clear whether
   UTF-8 and ISO-8859-1 coincide for all non-ASCII characters present)
 * in doc/info/extract_categories1.sed, guard against @" in captured strings

With these changes, ./configure --enable-lang-de --enable-foolisp
 $ make
has the expected effect of generating doc/info/de/maxima.info*
and doc/info/de/maxima*.html.

Also in this commit: update doc/info/common-lang.mk to look for
indexable stuff using pattern which works for Texinfo 6.
See target contents.hhc in doc/info/common-lang.mk.