doc: support html post processing in all cases

doc: support html post processing in all cases

Previously the html-local make rule only worked for `make html`.
Instead add support for `make doc/coreutils.html` or `make web-manual`
through the use of a makeinfo wrapper.

* doc/local.mk: Move post processing from here to ...
* build-aux/makeinfo-wrapper.sh: ... here.
* cfg.mk: Ensure our wrapper is called with MAKEINFO.
Also pass --no-node-files so redirection html files
are not created for each anchor.