systemctl: restore "systemctl reboot ARG" functionality

Commit d85515edcf changed logic how reboot is
executed. That commit changed behavior to use emergency action reboot code path
to perform the reboot.

This inadvertently broke rebooting with argument:
$ systemctl reboot custom-reason

Restore original behavior so that if reboot service unit similar to
systemd-reboot.service is executed it is possible to override reboot reason
with "systemctl reboot ARG".

When "systemctl reboot ARG" is executed ARG is placed in file
/run/systemd/reboot-param and reboot is issued using logind's Reboot
dbus-service.

If RebootArgument is specified in systemd-reboot.service it takes precedence
over what systemctl sets.

Fixes: #11828

src/core/emergency-action.c

@@ -47,7 +47,7 @@ void emergency_action(
         case EMERGENCY_ACTION_REBOOT:
                 log_and_status(m, warn, "Rebooting", reason);
 
-                (void) update_reboot_parameter_and_warn(reboot_arg);
+                (void) update_reboot_parameter_and_warn(reboot_arg, true);
                 (void) manager_add_job_by_name_and_warn(m, JOB_START, SPECIAL_REBOOT_TARGET, JOB_REPLACE_IRREVERSIBLY, NULL);
 
                 break;
@@ -55,7 +55,7 @@ void emergency_action(
         case EMERGENCY_ACTION_REBOOT_FORCE:
                 log_and_status(m, warn, "Forcibly rebooting", reason);
 
-                (void) update_reboot_parameter_and_warn(reboot_arg);
+                (void) update_reboot_parameter_and_warn(reboot_arg, true);
                 m->objective = MANAGER_REBOOT;
 
                 break;

src/shared/reboot-util.c

@@ -12,10 +12,13 @@
 #include "umask-util.h"
 #include "virt.h"
 
-int update_reboot_parameter_and_warn(const char *parameter) {
+int update_reboot_parameter_and_warn(const char *parameter, bool keep) {
         int r;
 
         if (isempty(parameter)) {
+                if (keep)
+                        return 0;
+
                 if (unlink("/run/systemd/reboot-param") < 0) {
                         if (errno == ENOENT)
                                 return 0;

src/shared/reboot-util.h

@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
-int update_reboot_parameter_and_warn(const char *parameter);
+int update_reboot_parameter_and_warn(const char *parameter, bool keep);
 
 typedef enum RebootFlags {
         REBOOT_LOG      = 1 << 0, /* log about what we are going to do and all errors */

src/systemctl/systemctl.c

@@ -3735,7 +3735,7 @@ static int start_special(int argc, char *argv[], void *userdata) {
                 return r;
 
         if (a == ACTION_REBOOT && argc > 1) {
-                r = update_reboot_parameter_and_warn(argv[1]);
+                r = update_reboot_parameter_and_warn(argv[1], false);
                 if (r < 0)
                         return r;
 
@@ -8444,7 +8444,7 @@ static int halt_parse_argv(int argc, char *argv[]) {
                 }
 
         if (arg_action == ACTION_REBOOT && (argc == optind || argc == optind + 1)) {
-                r = update_reboot_parameter_and_warn(argc == optind + 1 ? argv[optind] : NULL);
+                r = update_reboot_parameter_and_warn(argc == optind + 1 ? argv[optind] : NULL, false);
                 if (r < 0)
                         return r;
         } else if (optind < argc)