[settings] Merge SETTING_IPv4 and SETTING_IPv6

Allow for equivalent IPv4 and IPv6 settings (which requires equivalent
settings to be adjacent within the settings list).

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[dhcpv6] Add DHCPv6 "filename" setting

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[ipv6] Expose NDP-provided settings (including the DNS server)

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