net: dns: Add resolve API

This commit introduces a generic DNS resolving API that can
be used by applications. Later commits will introduce a system
level DNS support which simplifies the DNS resolving so that
DNS server names can be given from config file.

Change-Id: I60fbc81e2a44928d2ca53d51e703b9cde222b382
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>