Introduce /usr/lib/systemd/systemd-sysv-install [--root=] <action> <name>
abstraction, replacing the direct calling of chkconfig. This allows
distributions to call their specific tools like update-rc.d without patching
systemd.
Ship systemd-sysv-install.SKELETON as an example for packagers how to implement
this.
Drop the --enable-chkconfig configure option.
Document this in README and point to it in NEWS.