net/nbuf: Introduce the net_nbuf_linear_copy routine

This commit adds one routine previously found at the DNS resolver
library. The net_nbuf_linear_copy routine allows to "linearize"
an IP stack network buffer. This routine is required by functions
that must jump between big chunks of data that in this case may
lie between many fragments. Tracking fragments may be a tedious
task, so getting a linear copy of the buffer will reduce code
complexity altough it increases memory consumption.

The DNS client library is updated to reflect these changes.

Change-Id: Iae321f99fa9b05fae7e722b6d41baac427d82d7e
Signed-off-by: Flavio Santes <flavio.santes@intel.com>