Detect if SELinux is disabled and warn in e.g. rpm-ostree status

[jlebon]

rpm-ostree/ostree not working well with SELinux being temporarily disabled has come up multiple times recently. Until we officially support this and have CI for it, we may want to detect that case and print something in e.g. rpm-ostree status and possibly refuse to e.g. pull new OSTree content or import RPM packages without a --force switch or something.

In most cases, what users actually want is temporarily setting to permissive mode (enforcing=0) so we could mention that too.