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/maxscale.h

@@ -1,5 +1,6 @@
-#ifndef _MAXSCALE_H
-#define _MAXSCALE_H
+#pragma once
+#ifndef _MAXSCALE_MAXSCALE_H
+#define _MAXSCALE_MAXSCALE_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -27,8 +28,10 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <time.h>
 
+MXS_BEGIN_DECLS
 
 /* Exit status for MaxScale */
 #define MAXSCALE_SHUTDOWN       0   /* Good shutdown */
@@ -43,4 +46,6 @@ void maxscale_reset_starttime(void);
 time_t maxscale_started(void);
 int maxscale_uptime(void);
 
+MXS_END_DECLS
+
 #endif