shared: get rid of fileio-label.[ch]

Move the renaming function to reboot-util.h (since it writes out
/run/nologin at shutdown), and let's get rid of fileio-label.[ch] now
that it serves no purpose anymore.

src/shared/reboot-util.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <stdbool.h>
+
 bool reboot_parameter_is_valid(const char *parameter);
 int update_reboot_parameter_and_warn(const char *parameter, bool keep);
 
@@ -16,3 +18,5 @@ int reboot_with_parameter(RebootFlags flags);
 bool shall_restore_state(void);
 
 bool kexec_loaded(void);
+
+int create_shutdown_run_nologin_or_warn(void);