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.
@cpuguy83 what I was trying to say, is that I believe that @rhatdan's parsing code is buggy.  I believe that if a person were to run:
````
docker run -v /foo:/bar:rp ubuntu
````

then they would end up mounting the volume `/foo` as read-write, despite the fact that they had been trying to mount int read only (typo, switching o and p).  Because this code doesn't check for errors and would just happily default to being read-write.  That is what seems bad to me.