Investigate the use of cgroups to replace FUSE (and open lots of possibilities)

Given that tools like podman/buildah/... can mount images within cgroups without root permissions and without FUSE, this may serve as a viable alternative to using FUSE at all. Using cgroups just requires a relatively recent kernel, which most stable distributions nowadays have.

Using cgroups would allow us a lot of fancy additions. For instance, most sandboxing techniques rely on cgroups anyway. Another feature I can think of is writable overlays for AppImages (i.e., using something like aufs or overlayfs to mount a writable layer above the AppImage's filesystem, which, however, seems like some edge case feature to convince existing software to work when packaged into an AppImage, too).

Idea from https://github.com/AppImage/AppImageKit/issues/877#issuecomment-1637217703.