units: introduce network-pre.target as place to hook in firewalls

network-pre.target is a passive target that should be pulled in by
services that want to be executed before any network is configured (for
example: firewall scrips).

network-pre.target should be ordered before all network managemet
services (but not be pulled in by them).

network-pre.target should be order after all services that want to be
executed before any network is configured (and be pulled in by them).