org: adjust to org-mode 9.1
This is needed so that SVG files are included as an <object...> rather than as an <img...>, which in turn is needed to ensure SVG tooltips will work. We do not explicitly require org-mode 9.1, but we install it if it is not present. * HACKING: Mention the requirement. * doc/org/.dir-locals.el.in, doc/org/init.el.in, doc/org/spot.css: Adjust to org-mode 9.1. * doc/Makefile.am: Run emacs with the site-lisp libraries, in case it contains a more recent org-mode. * elisp/ob-dot.el: Delete, this was a work around older versions. * elisp/Makefile.am: Adjust.