resolver: Use proper IPv6 source address when sending DNS queries

resolver: Use proper IPv6 source address when sending DNS queries

This fix is for following scenario:
 - New interface is coming up.
 - There is radvd in the connected network and it is
   advertising IPv6 addresses and DNS servers.
 - Kernel receives router advertisement and picks up the DNS
   server information which is then routed via netlink to
   rtnl.c:rtnl_newnduseropt() which then creates DNS listener.
 - Kernel activates DAD (duplicate address detection).
 - As the DAD takes some time we now have interface up
   and it only has link local IPv6 address defined.
 - The DNS listener is now using link local source addresses when
   sending queries instead of proper autoconfigured addresses.
 - When DAD is finished, the interface will have autoconfigured
   addresses assigned and corresponding netlink message will cause
   function rtnl.c:process_newaddr() to be called.
 - If all this happens, then we re-create DNS listener
   in dnsproxy.c so that listener will have a proper
   (autoconfigured) source address when sending DNS packets.