shutdown: copy shutdown verb so shutdown scripts receive $1

The arg_verb was copying the pointer to the shutdown verb, but that
pointer was being overwritten. When the shutdown scripts run, the forked
child calls rename_process() which overwrites argv, so the scripts got
an empty $1 instead of reboot, poweroff, etc.

Make a copy of the verb with strdup() so it is not affected.

Fixes: #44087

Follow-up for: 4927588993

src/shutdown/shutdown.c

@@ -58,10 +58,12 @@
 #define SYNC_TIMEOUT_USEC (10*USEC_PER_SEC)
 #define DEFAULT_MINIMUM_UPTIME_USEC (15U * USEC_PER_SEC)
 
-static const char *arg_verb = NULL;
+static char *arg_verb = NULL;
 static uint8_t arg_exit_code = 0;
 static usec_t arg_timeout = DEFAULT_TIMEOUT_USEC;
 
+STATIC_DESTRUCTOR_REGISTER(arg_verb, freep);
+
 COMMAND(
         "systemd-shutdown\0",
         "Execute the final phase of system shutdown.",
@@ -157,9 +159,11 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 OPTION_POSITIONAL:
-                        if (!arg_verb)
-                                arg_verb = opts.arg;
-                        else
+                        if (!arg_verb) {
+                                arg_verb = strdup(opts.arg);
+                                if (!arg_verb)
+                                        return log_oom();
+                        } else
                                 log_warning("Got extraneous argument, ignoring.");
                         break;
                 }