Be better at cleaning up leftover from earlier test runs

When running the test inside a docker container we sometimes are left with
leftover device nodes for device mapper devices that no longer exist.
We were panic:ing in this case, but with this change we just remove such
nodes.
Make sure we mark the libdevmapper /dev/mapper/control fd CLOEXEC

We do a hack to mark it such, because otherwise lxc-start will not
work.
Small fixes
Small fixes