If we are running in a user namespace, don't try to mknod as
it won't be allowed. libcontainer will bind-mount the host's
devices over files in the container anyway, so it's not needed.
The chrootarchive package does a chroot (without mounting /proc) before
its work, so we cannot check /proc/self/uid_map when we need to. So
compute it in advance and pass it along with the tar options.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
