basic: split out update_reboot_parameter_and_warn() into its own .c/.h files

This is primarily preparation for a follow-up commit that adds a common
implementation of the other side of the reboot parameter file, i.e. the
code that reads the file and issues reboot() for it.

src/basic/reboot-util.h

@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+int update_reboot_parameter_and_warn(const char *parameter);