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/dbusers.h

@@ -1,5 +1,6 @@
-#ifndef _DBUSERS_H
-#define _DBUSERS_H
+#pragma once
+#ifndef _MAXSCALE_DBUSERS_H
+#define _MAXSCALE_DBUSERS_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -13,9 +14,11 @@
  * Public License.
  */
 
+#include <maxscale/cdefs.h>
 #include <maxscale/service.h>
 #include <arpa/inet.h>
 
+MXS_BEGIN_DECLS
 
 /**
  * @file dbusers.h Extarct user information form the backend database
@@ -77,4 +80,6 @@ extern char *mysql_users_fetch(USERS *users, MYSQL_USER_HOST *key);
 extern int reload_mysql_users(SERV_LISTENER *listener);
 extern int replace_mysql_users(SERV_LISTENER *listener);
 
+MXS_END_DECLS
+
 #endif