networkd: don't hard depend on system bus

We may not have a dbus daemon in the initrd (until we can rely on kdbus). In
this case, simply ignore any attempts at using the bus. There is only one user
for now, but surely more to come.

In order to work reliably in the real root without kdbus, but at the same time
don't delay boot when kdbus is in use, order ourselves after dbus.service.