systemd: sysext and confext provide "alternate" fsroots

[jcpunk]

Upstream ticket : SELinuxProject/selinux#491

per : https://www.freedesktop.org/software/systemd/man/latest/systemd-confext.html

/etc/extensions/, /run/extensions/ and /var/lib/extensions/ can have a named extension that treats the contained directory as "/".

For example /var/lib/extensions/example/usr/local/myfile" would be automatically overlayed as "/usr/local/myfile". It would be helpful if selinux automatically treated /var/lib/extensions/exampleas similar to/so arestoreconwould set the right selinux labels for/usr/local/myfile`.

There is a similar set of paths for confext - /run/confexts/, /var/lib/confexts/, /usr/lib/confexts/ and /usr/local/lib/confexts/.

There are also directories where mutibility changes are stored.