net: Restructure network protocols

This commit restructures the network protocols. Changes applied are:

- Move lib/iot/ to subsys/net/lib
- Move network protocol headers to include/net
- Move lib/iot/zoap/link-format.h to include/net/zoap_link_format.h
  and link-format.c to zoap_link_format.c
- Move tests/iot/ to tests/net/lib/
- Adapt sample code
- Adapt build system
- Modify doxygen paths

Change-Id: I37085fa4cc76a8a8e19a499ecb4e87b451120349
Signed-off-by: Flavio Santes <flavio.santes@intel.com>