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/listmanager.h

@@ -1,5 +1,6 @@
-#ifndef _LISTMANAGER_H
-#define _LISTMANAGER_H
+#pragma once
+#ifndef _MAXSCALE_LISTMANAGER_H
+#define _MAXSCALE_LISTMANAGER_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -26,9 +27,12 @@
  * @endverbatim
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/spinlock.h>
 #include <maxscale/skygw_debug.h>
 
+MXS_BEGIN_DECLS
+
 struct dcb;
 
 /*
@@ -115,5 +119,6 @@ void list_map(LIST_CONFIG *list_config, bool (*callback)(void *, ...));
 list_entry_t *list_remove_first(LIST_CONFIG *list_config);
 list_entry_t *list_remove_last(LIST_CONFIG *list_config);
 
+MXS_END_DECLS
 
 #endif /* LISTMANAGER_H */