The fetch_setting() family of functions may currently modify the
definition of the specified setting (e.g. to add missing type
information).  Clean up this interface by requiring callers to provide
an explicit buffer to contain the completed definition of the fetched
setting, if required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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>