Fix handling of multiple DNS servers

dnsmasq_dns_server was renamed to dnsmasq_dns_servers to handle
multiple DNS servers. Depending on which package is being installed
either dnsmasq_dns_server or dnsmasq_dns_servers is needed to be set.
It doesn't hurt to set both though.
Merge pull request #133 from dirkmueller/release/roxy/master

Fix handling of multiple DNS servers