vdev_id: multi-lun disks & slot num zero pad

Add ability to generate disk names that contain both a slot number
and a lun number in order to support multi-actuator SAS hard drives
with multiple luns. Also add the ability to zero pad slot numbers to
a desired digit length for easier sorting.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Heller <matthew.f.heller@accre.vanderbilt.edu>
Closes #16603