dns: FIX: use byteorder.h functions

The previous version of the DNS Client sample code assumes a
little-endian machine as a target. In this patch we use the byteorder.h
functions to handle the cpu to network order conversions.
Theoretically, this code must run on little and big endian machines.

Cosmetic changes are also applied to other files.

Change-Id: I2b47f6252fa070e0a0253666fde1785530d6aebf
Signed-off-by: Flavio Santes <flavio.santes@intel.com>