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/adminusers.h.in

@@ -1,5 +1,6 @@
-#ifndef _ADMINUSERS_H
-#define _ADMINUSERS_H
+#pragma once
+#ifndef _MAXSCALE_ADMINUSERS_H
+#define _MAXSCALE_ADMINUSERS_H
 /*
  * Copyright (c) 2016 MariaDB Corporation Ab
  *
@@ -24,8 +25,12 @@
  *
  * @endverbatim
  */
+
+#include <maxscale/cdefs.h>
 #include <maxscale/dcb.h>
 
+MXS_BEGIN_DECLS
+
 #define ADMIN_SALT "$1$MXS"
 
 /* Max length of fields in for admin users */
@@ -63,4 +68,6 @@ extern bool        admin_verify_inet_user(const char *uname, const char *passwor
 
 extern void        dcb_PrintAdminUsers(DCB *dcb);
 
+MXS_END_DECLS
+
 #endif