net: dns: Allow to initialize DNS servers from struct sockaddr[]

In some cases, we need to initialize DNS servers from a binary
addresses, e.g. in case of DHCP processing. With existing API,
such addresses would need to be converted to strings, just to
be converted back to struct sockaddr in dns_resolve_init().
This is not efficient, and with a number of addresses quite
cumbersome. So instead, allow to pass DNS server either as
strings, or as struct sockaddr's (or both).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>