Add SMART self-test results to zpool status -c

Add in SMART self-test results to zpool status|iostat -c. This
works for both SAS and SATA drives.

Also, add plumbing to allow the 'smart' script to take smartctl
output from a directory of output text files instead of running
it against the vdevs.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7178