core/net/resolv: Switch default IPv6 DNS to use Google's Public DNS.

This was previously using HE's public DNS, but now that
Google has IPv6 support for their public DNS it seems
appropriate to use them as the default.

Note that this default only makes sense if the node has
access to the IPv6 internet. Nodes should provide a way
to change (or auto detect) the appropriate DNS server
in such cases.