net: dns: Bind DNS server to a network interface

Allow user to specify a network interface in the DNS server
list. User can append "%" and network interface name to the
DNS server to use this. If the network interface is mentioned
in the server list, then the DNS queries are sent via this network
interface.
For example setting the interfaces like this:
   192.0.2.2%eth1
   [2001:db8::2]:5353%ppp0

would cause the DNS queries to sent to 192.0.2.1 via eth1 in the first
example, and to 2001:db8::2 via ppp0 in the second example.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>