Don't return owner in list_owner_ranges API call.

Don't return owner in list_owner_ranges API call.

Closes: 339
struct subordinate_range is pretty closely tied to the existing
subid code and /etc/subuid format, so it includes an owner.  Dropping
that or even renaming it is more painful than I'd first thought.
So introduce a 'struct subid_range' which is only the start and
count, leaving 'struct subordinate_range' as the owner, start and
count.

Signed-off-by: Serge Hallyn <serge@hallyn.com>