This idea came from Andy Lutomirski. Instead of using a
temporary directory for the pivot_root put-old, use "." both
for new-root and old-root. Then fchdir into the old root
temporarily in order to unmount the old-root, and finally
chdir back into our '/'.
Drop lxc.pivotdir from the lxc.container.conf manpage.
Warn when we see a lxc.pivotdir entry (but keep it in the
lxc.conf for now).