These enable bubblewrap to create overlay mounts.  This will be useful for
an ostree-based build system we use where overlayfs ensures that none
of the ostree hard-linked files I checkout get modified.  Currently we
use a maze of bash/unshare/mount/sudo/chroot where bubblewrap will be much
nicer.

This commit contains a bit of string manipulation, which isn't
particularly fun to write in C.  Hopefully I got it right.

No tests are written to exercise this new feature.