net: dhcpv4: Honor network interface for DNS servers

Bind the network interface to the network interface we
have received the DNS servers from. This is now the default.
The previous behavior can be restored by disabling the
CONFIG_NET_DHCPV4_DNS_SERVER_VIA_INTERFACE option.

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