net: dns: Introduce resolver function that avoids using the cache

Introduce dns_resolve_name_internal() that allows resolving a name
and not use DNS cache if caching is enabled.

This is needed in mDNS probing (RFC 6762 chapter 8.1) which needs
to send a mDNS query and not get any results from cache.

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