base code related to coap header of 'CoAP Over TCP' in libcoap.

base code related to coap header of 'CoAP Over TCP' in libcoap.

- length field of coap header can be consist 8/16/32 bit
  with unsigned integer.
- https://wiki.iotivity.org/proposal_for_cloud_interface_in_iotivity

Change-Id: Ieedaa8d7812f0375e40d5d6e13c6f332d22d68bd
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2196
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Tested-by: Jon A. Cruz <jonc@osg.samsung.com>