revise systemd management arrangements

In discussion with systemd developers, i think i've sorted out an improvement over the existing multi-daemon systemd supervision arrangement.

The changes are not large, but they result in an easier-to-manage set of system services, and they abolish the use and distribution of symlinks, which i found was causing problems with the tarballs, and also tends to confuse both system administrators and packaging systems (e.g., dh_installsystemd has trouble with symlinks).

I've updated the documentation as well. I'm currently shipping something close to this arrangement in debian's 2.0.0-2 packaging of knot-resolver (in debian unstable), so hopefully if there are problems with it we'll hear about it. I was unable to get a package built with the earlier arrangement that behaved the way i was hoping it would behave :/