Allow caller to change the SELinux 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.

This pull request is split out from the pull request for docker

https://github.com/dotcloud/docker/pull/5910


Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
nit: This is only used in badPrefix(). Can we move it in that method, or are you expecting to reuse it?