Consider adapting "auto rootfs" support into core

For the case of targeting an arbitrary rootfs (most commonly the booted one), today our `demos/bubblewrap-shell.sh` looks for e.g. `/lib64` and other distribution-specific bits in an ad-hoc way.

Tools like flatpak and rpm-ostree only use bwrap targeting root filesystems constructed in a known way. But efforts like the non-flatpak GNOME thumbnailing sandbox wants to reuse the host rootfs, which could be arbitrary.

So this issue is about something like `--inherit-rootfs-usr /` which takes `/usr` but *also* knows how to pick up compatibility symlinks in `/` and handles split `/lib` if the distribution hasn't done [UsrMerge](https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/), and generates these things in the target tmpfs.