Draft: Sxmo dumb runtime dir

This change will make the XDG_RUNTIME_DIR to become /var/run/user/$id on Sxmo. Before that point, our /etc/profile.d/sxmo_init.sh was using a fallback to /dev/shm/user/$id. The problem is that this path is less common, and can cause problems. By example, flatpak refuse to bind mount this directory, located under /dev, and that can cause issues with some programs.

I think that dumb_runtime_dir is a more simple and maintained approach than pam-rundir. And now that I make it to works OOTB upstream, adding the dependency here is enough to use it (see https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/52520)