doc: html: support defined anchors for command options

doc: html: support defined anchors for command options

* doc/coreutils.texi (optAnchor): A new macro to output a
referencable anchor, called from ...
(optItem): ... here; a new macro to output all index entries
for each option item.
(optZero,optZeroTerminated): Show an example of the adjustment
done to each option description.
* doc/local.mk (html-local): Post-process the texinfo generated HTML
(`make html`) to remove our "-option" tag, and replace all
escaped _002d with a standard hyphen, which is fine in URLs.