user-manager still wants to write to `sddm.conf` and it's using sddm_kcm's function to do so, but this function no longer supports writing to `sddm.conf`
That means auto-login settings are no longer getting saved anywhere.
This patch tweaks user-manager to write to `/etc/sddm.conf.d/kde_setings.conf`, making the feature work again.
NOTE: The autologin checkbox will keep getting reset until we fix bug 414396 because the `kde_settings.conf` file is not world-readable at this moment.
BUG: 342722
FIXED-IN: 5.18.0