MXS-2495 Cleanup GWBUF interface

All GWBUF macros that address a single link in a chain are now
simple wrappers for equivalent gwbuf_link-functions.

Next step is to drop the macros and replace their use with calls
to the functions.

server/core/test/test_modutil.cc

@@ -593,7 +593,7 @@ void test_large_packets()
     {
         GWBUF* buffer = gwbuf_append(create_buffer(0x00ffffff), create_buffer(i));
         mxb_assert(gwbuf_length(buffer) == 0xffffffUL + i + 8);
-        GWBUF_RTRIM(buffer->next, 1)
+        GWBUF_RTRIM(buffer->next, 1);
         GWBUF* complete = modutil_get_complete_packets(&buffer);
         mxb_assert_message(buffer, "Incomplete buffer is not NULL");
         mxb_assert_message(complete, "The complete buffer is not NULL");