ipv6_ra_conf is incorrectly set in rdnssd when an ISP uses DHCPv6

Correctly set ipv6_ra_conf="mset" in the "managed" case. When ipv6_ra_conf is set to 'oset' the dhcp6c.conf file does not include an IA_NA section which results in dhcp6c not requesting an IPv6 WAN address.

This caused problems for at least one Singapore ISP and for Comcast users only resulted in a lack of IPv6 WAN address.