Allow caller to change the labels on a directory tree.

We want to add this to libcontainer so that we can change docker so that
when you volume mount into a labeled container, we want to allow the
administrator/user the ability to tell docker to fix the labels on the mount.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #47 from rhatdan/master

Allow caller to change the SELinux labels on a directory tree.