Warn on modified SELinux policy

[travier]

Feature Request

Environment

Fedora CoreOS

Desired Feature

Warn if the SELinux policy has been modified from the default and might thus be outdated after an update.

Other Information

Frequent source of SELinux denials / errors for users.

To figure out if the policy has been modified, we can use:

$ sudo ostree admin config-diff | grep selinux
...

[travier]

We've fixed the main problem with custom SELinux policies and thus we don't need this one anymore.

See coreos/fedora-coreos-tracker#701, which is not yet closed as we still need to add tests.