network-manager: Big overhaul

  * 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.