Cleanup header files

- All now include maxscale/cdefs.h as the very first file.
- MXS_[BEGIN|END]_DECLS added to all C-headers.
  Strictly speaking not necessary for private headers, but
  does not hurt either.
- Include guards moved to the very top of the file.
- #pragma once added.

server/core/test/test_utils.h

@@ -1,6 +1,6 @@
+#pragma once
 #ifndef TEST_UTILS_H
 #define TEST_UTILS_H
-
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -14,6 +14,7 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/poll.h>
 #include <maxscale/dcb.h>
 #include <maxscale/housekeeper.h>