This updates rdnssd to the following:
* Using the systemd interfaces directly
* Using the rdnssd user instead of the root user
* Integrating with resolvconf instead of writing directly to /etc/resolv.conf
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
