Add a section about ZFS self-healing.  An example is shown where a
mirrored pool is intentionally corrupted (with a big warning sign)
and how ZFS copes with it.

This is based on an example I did for lecture slides I created back
in the days when the links to www.sun.com where still in the output
of zpool status.  This needs to be updated later with the links
that are displayed now.