bug#41413: guix-install.sh broken on Gentoo

[ZC via web <issues.guix.gnu.org@elephly.net>]

guix-install.sh calls "update-rc.d", which doesn't exist on a Gentoo system. Instead of trying to support all possible init systems, or make assumptions regarding how the init system works, I'd suggest simply dropping support for anything other than upstart and/or systemd. Users of smaller distros using unusual/traditional init systems can add support for guix theirself. The safest and most portable option would likely be to simply point to the init script at the end of the install, and tell the user to copy that file to the proper directory and enable it, copy and modify it to fit their system, or use it as a template for whatever their init system happens to be. It's less user-friendly in the "run it and forget it" sense, but it won't break a user's already working system, or error out in new and fun ways.