libsystemd-network: add support for "Client FQDN" DHCP option

This adds support for the Client Fully Qualified Domain Name (FQDN)
option [RFC 4702] to libsystemd-network. The option can be used to
exchange information about a DHCPv4 client's fully qualified domain
name and about responsibility for updating the DNS RR related to the
client's address assignment.

Other popular DHCP clients (dhclient, dhcpcd) support this option and
it would be useful to have it in networkd too.