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.

include/maxscale/modinfo.h

@@ -1,5 +1,6 @@
-#ifndef _MODINFO_H
-#define _MODINFO_H
+#pragma once
+#ifndef _MAXSCALE_MODINFO_H
+#define _MAXSCALE_MODINFO_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -25,6 +26,10 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
+
+MXS_BEGIN_DECLS
+
 /**
  * The status of the module. This gives some idea of the module
  * maturity.
@@ -83,4 +88,7 @@ typedef struct
     MODULE_VERSION  api_version;
     char        *description;
 } MODULE_INFO;
+
+MXS_END_DECLS
+
 #endif