When bind-mounting the tree for i/o, use recursive bind mounts.
This could be needed in the case that `/usr` is not one single
mount but assembled from different ones. Normally this should
not be the case but we want to support in, just in case.
Conversely, when unmounting, do so recursively too.
NB: This should not make any differences for trees that we have
built ourselves since they don't contain any mounts.