mnexec: properly setup the mount namespace
Systemd's default is to mark the root mount as shared and it is
inherited as such by the new mount namespace. This means that any
mounts performed inthe new namespace will be visible by the rest of
the system, breaking privateDirs.
To restore a more sane behaviour, we explicitly mark all mounts
recursively as private, meaning that we will no longer see new mounts
from the root namespace, and our mounts will also not propagate to the
rest of the system.