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/users.h

@@ -1,5 +1,6 @@
-#ifndef _USERS_H
-#define _USERS_H
+#pragma once
+#ifndef _MAXSCALE_USERS_H
+#define _MAXSCALE_USERS_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -12,11 +13,15 @@
  * of this software will be governed by version 2 or later of the General
  * Public License.
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/hashtable.h>
 #include <maxscale/dcb.h>
 #include <maxscale/listener.h>
 #include <openssl/sha.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * @file users.h The functions to manipulate the table of users maintained
  * for each service
@@ -70,4 +75,6 @@ extern int users_default_loadusers(SERV_LISTENER *port); /**< A generic implemen
 extern void usersPrint(USERS *);                  /**< Print data about the users loaded */
 extern void dcb_usersPrint(DCB *, USERS *);       /**< Print data about the users loaded */
 
+MXS_END_DECLS
+
 #endif