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.
I think that it would be better to make Mode into a proper data type, and create a function to turn it back into a string in the LXC code, rather than passing around a string that just happens to be passable to the `Writtable` and `MakeReadOnly` functions.