Notify on UNAVAIL statechange

UNAVAIL is maybe not quite as concerning as DEGRADED, but still an
event of notice, in my opinion. For example it is triggered when a
drive goes missing.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Francesco Mazzoli <f@mazzo.li>
Closes #12629
Closes #12630