Don't explicitly load policy module, which is loaded by default anyway.

Previously we were using names "Knot DNS Resolver" and "Knot Resolver"
interchangibly and the prefix "Knot DNS" was somehow confusing users.
Let's see if this rebranding actually helps or not.

Starting multiple systemd units with a target is conceptually cleaner
than using globbing expression.

Related https://github.com/systemd/systemd/issues/9080