namespace-util: set mounts back to MS_SHARED in detach_mount_namespace()

namespace-util: set mounts back to MS_SHARED in detach_mount_namespace()

For nspawn and services we first turn off two-way propagation of mounts
from host to sandbox via MS_SLAVE, and then set MS_SHARED again, so that
we create a new mount prop peer group again, and that we provide
behaviour similar to what we provide on the host further down the tree.

Let's do the same in detach_mount_namespace(), which we use for the
temporary mounts in the implementation of --image= in various tools.

This doesn't fix any immediate issue, but ensures we expose somewhat
systematic behaviour: whenever we detach mount namespaces we always set
things back to MS_SLAVE in the child.