dnsmasq: Add conntrack support in the full variant

Conntrack support reads the connection track mark associated with
incoming DNS queries and sets the same mark value on the upstream
forwarded DNS query. This can be usefull to track traffic generated
by dnsmasq to associate it with the clients who generate the queries,
usefull for bandwidth accouting and firewall.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>