Replace node-menuify.py with an invocation of Emacs

This obviates the hand-scripting, instead invoking Emacs in batch mode to update menus. Doing this also removes the limitation that the script was only able to update existing menus, not to insert missing menus.

While at it, add --no-init-file --no-site-file to the Emacs invocation in fixscm.sh. These options prevent loading user configuration Lisp code, which reduces logging output and should ensure that the result does not depend on the settings of the developer running the script.