vdev_id: support daisy-chained enclosures

Disks in enclosures in daisy-chained configurations will currently get
conflicting names when using the sas_direct and sas_switch topologies.
This is because the "channel" keyword syntax lacks sufficient location
information to distinguish between enclosures connected the same
physical port.  The channel keyword now supports an optional numeric
enclosure_id parameter to identify the position of an enclosure in a
daisy-chained configuration.  Daisy-chained enclosures are numbered
starting from 0.

Signed-off-by: Ned Bass <bass6@llnl.gov>

Fixes #2074