- Use DefaultDependencies=no in ifup@.service so the service can be started
  as early as possible.
- Create the ifupdown runtime directory in ifup@.service as we can no longer
  rely on the networking service to do that for us.
- Don't stop ifup@.service on shutdown but let the networking service take
  care of stopping all hotplugged interfaces.
- Only start ifup@.service for interfaces configured as allow-hotplug.