Allow user to poweroff/reboot from greetd/tuigreet

(Coming from https://community.mnt.re/t/debian-bookworm-stable/2529/5 )

Originally my Pocket using debian/stable could not reboot/poweroff from greetd/tuigreet. To enable it I set the --power-shutdown and --power-reboot on /etc/greetd/config.toml:

command = "/usr/bin/tuigreet --window-padding 4 --remember --asterisks --cmd /usr/bin/sway --power-shutdown 'systemctl poweroff' --power-reboot 'systemctl reboot'"

I think that this could also apply to stock debian/sid.