net/dns: Introduce the qname_copy routine

When a CNAME is part of the DNS answer RR, sometimes a label with
a pointer is found. The CNAME must be reused to create a new DNS
query and that CNAME will become the new DNS Query QNAME. This new
QNAME must not include pointers.

This patch introduces the qname_copy routine that "linearizes" a
given QNAME (perhaps with pointers).

The dns_read routine is also updated to reflect these changes.

Change-Id: I8e8f64e85e2cbf494fd589e2b7a67d470d34604b
Signed-off-by: Flavio Santes <flavio.santes@intel.com>