zed: Add zedlet to power off slot when drive is faulted

If ZED_POWER_OFF_ENCLOUSRE_SLOT_ON_FAULT is enabled in zed.rc, then
power off the drive's slot in the enclosure if it becomes FAULTED.
This can help silence misbehaving drives. This assumes your drive
enclosure fully supports slot power control via sysfs.

Reviewed-by: @AllKind
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15200