Fix DNS propagation when run via NetworkManager

It got broken in ebac728f

Although from resolver perspective v4 and v6 entries in /etc/resolv.conf doesn't differ in format, it does make difference for NetworkManager. Passing v6 address in v4 related variables broke updating resolv conf what effects in lack of DNS propagation when there's at least one v6 DNS address returned from the VPN endpoint.

https://github.com/NetworkManager/NetworkManager/blob/master/src/vpn/nm-vpn-connection.c#L1533-L1536

This change introduced clear distinguish between those two.

This is also supported in default vpnc-script which is shipped in Arch distribution, I assume it's similar for other distros:

 #* INTERNAL_IP6_DNS             -- IPv6 list of dns servers