Implement support for `-Sdd`

With systemd-sysext it might be usefull to bootstrap a chroot with packages that doesn't have the completely dependency graph installed along with it.

https://man.archlinux.org/man/systemd-sysext.8.en

Should just be a fairly trivial extension of the patch adding -U support.

https://github.com/archlinux/arch-install-scripts/commit/e1a84d105699969a41136ddc0b3f7816ae907bfc