Includes: Remove unnecessary include files

Include common system header files in coap_internal.h based on HAVE_*H
definitions.

Remove these system header files from include/*/*.h.

Add in system header files where appropriate in various apps.

examples/zephyr/test-cpp-src/src/test.cpp

@@ -17,5 +17,5 @@ coap_str_const_t *xxx_libcoap_get_version() {
 int main()
 {
   xxx_libcoap_get_version();
-  exit(0);
+  return 0;
 }