sd-dhcp6-client: Add functions to bind to DHCPv6 UDP socket

sd-dhcp6-client: Add functions to bind to DHCPv6 UDP socket

Add a function that creates a UDP socket bound to the given interface
and optionally to an IPv6 address. Add another function that will
send the DHCPv6 UDP packet to its destination.

Using IPV6_PKTINFO in setsockopt to bind the IPv6 socket to an
interface is documented in section 4. of RFC 3542, "Advanced Sockets
Application Program Interface (API) for IPv6"

Add a define for DHCPv6 Relay Agents and Servers multicast address as
its not available elsewhere.