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/systemctl/systemctl.c

@@ -73,6 +73,7 @@
 #include "path-util.h"
 #include "process-util.h"
 #include "raw-reboot.h"
+#include "reboot-util.h"
 #include "rlimit-util.h"
 #include "set.h"
 #include "sigbus.h"