networkd: Allow to retain configs even if carrier is lost

networkd: Allow to retain configs even if carrier is lost

When there is bad link in the network the carrier goes up/down.
This makes networkd stops all the clients and drop config.
But if the remote router/dhcpserver running a prevention
of DHCP Starvation attack or DHCP Flood attack it does not allow
networkd to take a DHCP lease resulting failure in configuration.
This patch allows to keep the client running and keep the conf
also for this scenario.

Closes #9111