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/modules/include/debugcli.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _DEBUGCLI_H
 #define _DEBUGCLI_H
 /*
@@ -13,10 +14,14 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/service.h>
 #include <maxscale/session.h>
 #include <maxscale/spinlock.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * @file debugcli.h The debug interface to the gateway
  *
@@ -63,4 +68,7 @@ typedef struct cli_session
 /* Command line interface modes */
 #define CLIM_USER       1
 #define CLIM_DEVELOPER      2
+
+MXS_END_DECLS
+
 #endif