This patch adds SELinux labeling support.

docker will run the process(es) within the container with an SELinux label and will label
all of  the content within the container with mount label.  Any temporary file systems
created within the container need to be mounted with the same mount label.

The user can override the process label by specifying

-Z With a string of space separated options.

-Z "user=unconfined_u role=unconfined_r type=unconfined_t level=s0"

Would cause the process label to run with unconfined_u:unconfined_r:unconfined_t:s0"

By default the processes will run execute within the container as svirt_lxc_net_t.
All of the content in the container as svirt_sandbox_file_t.

The process mcs level is based of the PID of the docker process that is creating the container.

If you run the container in --priv mode, the labeling will be disabled.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #4211 from rhatdan/master

Add SELinux support