networkd: Allow DHCP servers to be re-configured on carrier gain

networkd: Allow DHCP servers to be re-configured on carrier gain

In normal operation this would trigger an assertion
when a DHCP server is configured every time the link goes up.

This change makes sd_dhcp_server_configure_pool idempotent
and stops the DHCP server when the link loses carrier.

In addition to this stopping the assertion being triggered,
this has the useful side-effect of allowing the link to be taken down
and then brought back up as a way to have it use DNS from an "upstream"
interface that got its DNS configuration via DHCP
after the downstream link was configured.