login: make sure to replace existing units

When queuing unit jobs, we should rather replace existing units than
fail. This is especially important when we queued a user-shutdown and a
new login is encountered. In this case, we better raplce the shutdown
jobs. systemd takes care of everything else.

src/login/logind-dbus.c

@@ -2964,7 +2964,7 @@ int manager_start_unit(Manager *manager, const char *unit, sd_bus_error *error,
                         "StartUnit",
                         error,
                         &reply,
-                        "ss", unit, "fail");
+                        "ss", unit, "replace");
         if (r < 0)
                 return r;