resolvconf service: init

This is a refactor of how resolvconf is managed on NixOS. We split it
into a separate service which is enabled internally depending on whether
we want /etc/resolv.conf to be managed by it. Various services now take
advantage of those configuration options.

We also now use systemd instead of activation scripts to update
resolv.conf.

NetworkManager now uses the right option for rc-manager DNS
automatically, so the configuration option shouldn't be exposed.