sd-dhcp6-client: Implement Information Request message

sd-dhcp6-client: Implement Information Request message

Implement Information Request message according to RFC 3315, section
18.1.5. with the excepion that the first message is not delayed by a
random amount. Instead systemd-networkd is supposed to take care of
desynchronizing between other clients.

Initialize the DHCPv6 client structure in sd_dhcp6_client_start()
as this allows toggling between information request and normal
DHCPv6 address aquisition modes.