networkd: Be opportunistic when declaring link configured
The DHCP variable in the [Network] section of the network configuration
file moves the link to the configured state according to the following:
DHCP=yes Link is configured when either DHCPv4 or DHCPv6 succeed
DHCP=ipv4 Link is configured only when DHCPv4 succeeds
DHCP=ipv6 Link is configured only when DHCPv6 succeeds
DHCP=no Neither DHCPv4 or DHCPv6 is attempted