IPv6 Prefix Delegation fixes

After receiving an IPv6 prefix via DHCPv6-PD, the prefix is not
advertised until dhcp6c-state is manually called in the router.

If DHCPv6-PD is enabled and no DNS is acquired from the router
advertisement, try to retrieve the DNS via the DHCPv6 client.
Don't advertise IPv6 MTU for native interfaces

IPv6 tunnel interfaces have a 20 byte overhead (IPv4 header size).
Native IPv6 does not have this limitation.
Remove case IPV6_NATIVE_DHCP: before default:

As suggested in the pull request, removing the explicit IPV6_NATIVE_DHCP case and letting the default case handle it.
Always request DNS servers via dhcp6c

As proposed in the pull request, always request DNS servers without looking at ipv6_ra_conf for now.
Merge pull request #548 from pinwing/master

IPv6 Prefix Delegation fixes