libcontainer: Detect availablity, and do not do containers on RHEL6

At the moment, bodhi in Fedora is trying to run rpm-ostree inside a
mock chroot on RHEL6, and it's failing with the unshare(/) + remount
bit.  This does work with a clone(), but using that would require
some rearchitecting.

Let's just not do container stuff if we detect this situation.