manager: use reboot.target as the default for ctrl-alt-del.target

systemd tries to start ctrl-alt-del.target after receiving a request
to reboot in a container (or when ctrl-alt-del is pressed on a real
machine). If the alias is missing, this will fail. Let's provide a
fallback for this too. I don't think it makes sense to make it
configurable: it seems almost everybody is fine with the default
mapping. If not, people can always just create the symlink in the
filesystem.

src/basic/special.h

@@ -110,6 +110,9 @@
 #define SPECIAL_SIGPWR_TARGET "sigpwr.target"
 #define SPECIAL_CTRL_ALT_DEL_TARGET "ctrl-alt-del.target"
 
+/* The unit to use as fallback if ctrl-alt-del.target (which is usually an alias) cannot be loaded. */
+#define FALLBACK_CTRL_ALT_DEL_TARGET "reboot.target"
+
 /* Where we add all our system units, users and machines by default */
 #define SPECIAL_SYSTEM_SLICE "system.slice"
 #define SPECIAL_USER_SLICE "user.slice"

src/core/manager.c

@@ -3220,10 +3220,10 @@ turn_off:
         return 0;
 }
 
-static void manager_start_special(Manager *m, const char *name, JobMode mode) {
+static void manager_start_special_with_fallback(Manager *m, const char *name, const char *fallback, JobMode mode) {
         Job *job;
 
-        if (manager_add_job_by_name_or_warn(m, JOB_START, name, mode, NULL, &job) < 0)
+        if (manager_add_job_by_name_or_warn_with_fallback(m, JOB_START, name, fallback, mode, /* affected_jobs= */ NULL, &job) < 0)
                 return;
 
         const char *s = unit_status_string(job->unit, NULL);
@@ -3235,6 +3235,10 @@ static void manager_start_special(Manager *m, const char *name, JobMode mode) {
         m->status_ready = false;
 }
 
+static void manager_start_special(Manager *m, const char *name, JobMode mode) {
+        manager_start_special_with_fallback(m, name, /* fallback= */ NULL, mode);
+}
+
 static void manager_handle_ctrl_alt_del(Manager *m) {
         assert(m);
 
@@ -3242,7 +3246,11 @@ static void manager_handle_ctrl_alt_del(Manager *m) {
          * unless it was disabled in system.conf. */
 
         if (ratelimit_below(&m->ctrl_alt_del_ratelimit) || m->cad_burst_action == EMERGENCY_ACTION_NONE)
-                manager_start_special(m, SPECIAL_CTRL_ALT_DEL_TARGET, JOB_REPLACE_IRREVERSIBLY);
+                manager_start_special_with_fallback(
+                                m,
+                                SPECIAL_CTRL_ALT_DEL_TARGET,
+                                FALLBACK_CTRL_ALT_DEL_TARGET,
+                                JOB_REPLACE_IRREVERSIBLY);
         else
                 emergency_action(
                                 m,