Pass both IPv4 and IPv6 nameservers to dnsmasq

Additionally, store and return a list of strings for
DNS forwarders, since that's what is most useful. These
strings are guaranteed to be valid IP string literals by
virtue of validation on input.

Bug: 9580643
Change-Id: I4fbaf19835055b12e38d585c0bfba0d2b18b9717