DHCPv6 client: IPv6 address autoconfiguration as per RFC 3315 (stateful DHCPv6) and RFC 3736 (stateless DHCPv6).

For now, only stateless DHCPv6 is implemented!

TODO:

• enable/disable API to not always start when RA is received
• stateful DHCPv6 (for now, only stateless DHCPv6 for DNS and NTP servers works)
• create Client Identifier?
• only start requests if a valid local address is available on the netif
• only start information requests if required (not for every RA)

dhcp6_enable_stateful() enables stateful DHCPv6 for a netif (stateless disabled)
dhcp6_enable_stateless() enables stateless DHCPv6 for a netif (stateful disabled)
dhcp6_disable() disable DHCPv6 for a netif

When enabled, requests are only issued after receipt of RA with the corresponding bits set.