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/service.h

@@ -1,5 +1,6 @@
-#ifndef _SERVICE_H
-#define _SERVICE_H
+#pragma once
+#ifndef _MAXSCALE_SERVICE_H
+#define _MAXSCALE_SERVICE_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -13,6 +14,7 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
 #include <time.h>
 #include <maxscale/gw_protocol.h>
 #include <maxscale/spinlock.h>
@@ -53,6 +55,9 @@
  *
  * @endverbatim
  */
+
+MXS_BEGIN_DECLS
+
 struct server;
 struct router;
 struct router_object;
@@ -222,4 +227,6 @@ extern RESULTSET *serviceGetList();
 extern RESULTSET *serviceGetListenerList();
 extern bool service_all_services_have_listeners();
 
+MXS_END_DECLS
+
 #endif