l2hinit.perl version of the function (which
		     overrode a version in latex2html), but
		     do_cmd_index() was overridden.  Since
		     do_cmd_index() was the only caller, we can trash
		     this.

define_indexing_macro():  New function used during setup to support
			  more dynamic dispatch for indexing.

process_index_macros():  New function; implement dynamic dispatching
			 of indexing support functions, allowing many
			 targets used in indexing to be shared by
			 multiple index entries.  I.e., something like
			 \index{foo}\bifuncindex{bar} can now share a
			 single target anchor.

These indexing changes reduce the amount of clutter in the generated
HTML files.  In the reference manual, this cuts the number of targets
by 44%.  Targets are less affected when generated by environments
rather than macros.