networkd: work inside containers

Udev does not run in containers, so instead of relying on it to tell us when a
network device is ready to be used by networkd, we simply assume that any
device was fully initialized before being added to the container.
sd-dhcp: make sure client->secs > 0

Some DHCP servers will not work correctly if secs == 0, so round up
to at least 1.