* Add group 'networkmanager' and implement polkit configuration
that allows users in this group to make persistent, system-wide
changes to NetworkManager settings.
* Add support for ModemManager. 3G modems should work out of the
box now (it does for me...). This introduces a dependency on
pkgs.modemmanager.
* Write NetworkManger config file to Nix store, and let the
daemon use it from there.