Various ifupdown integration fixes
- 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.