Add support for the stateful and stateless variants of the DHCPv6
protocol. The resulting settings block is registered as
"net<x>.dhcpv6", and DHCPv6 options can be obtained using
e.g. "${net0.dhcpv6/23:ipv6}" to obtain the IPv6 DNS server address.
IPv6 addresses obtained via stateful DHCPv6 are not yet applied to the
network device.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
