Detail: show which 'set' each device in a RAID10 belongs to

Detail: show which 'set' each device in a RAID10 belongs to

A RAID10 can be though of as having 2 sets of devices
(if there are 2 copies and an even number of devices in total).

With this patch "mdadm --detail" shows which 'set' each device
belongs to - set-A or set-B.

If there are more than 3 copies, there can be more than 3 sets.

If the number of copies does not evenly divide the number of devices,
there are not distinct 'sets' so none are reported.

Signed-off-by: NeilBrown <neilb@suse.de>