Modify volume mounts SELinux labels on the fly based on :Z or :z

Looking for comment on this patch.  

The first patch should be able to be applied without any controversy.  The send patch is extending the qualifiers on the -v command to allow an admin to tell the system to relabel, content.  There might be a need for something similar for changing the DAC Permissions.
LGTM, nicely done. (Don't forget to capitalize Docker!). Thanks!