admin: selinux-ensure-labeled: new builtin

Code like rpm-ostree generates disk images directly.  In order to
ensure SELinux labeling is correct, it currently has a helper program
that runs over the deployment root, then over the whole disk and to
only set a label if none exist.

In order to make it easier to write installers such as Anaconda
without having them depend on rpm-ostree (or whatever other
build-server side program), pull in the helper code here.