…ture

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

- use an intermediate serviceNetworks slice so that we don't have
  to call service.NetworksByPriority multiple times.
- shift the primary network from the slice (if any), so that
  we can drop some checks for "additional networks"
- group code related to setting up the primary network as first
  step, then append remaining networks.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>