struct kset is what struct subsystem should have originally been called. It
is a set of kobjects, and nothing more, with a much less confusing name than
'subsystem.'

struct kset contains an embedded kobject, making it possible to represent it
in the object hierarchy, and sysfs. This also provides a means for objects
to easily express a list of subordinate objects. 

struct subsystem still exists, and contains an rwsem, which its subordinate
ksets use to protect their lists. 

An arbitrary number of ksets may belong to a subsystem. A ksets specifies
the subsystem it belongs to via its ->subsys field. 

struct subsystem also contains a default kset, which may be used without
having to define a separate kset. 

The objects that defined subordinate subsystems (bus and class drivers) have
been converted to use subordinate ksets instead.


Note that the usage of ksets is flexible. 
- ksets may contain a list of objects of any type, not just kobjects.
- The objects registered with a kset do not have to be registered.
- ksets themselves do not have to be registered. One can be used by
  simply calling kset_init(). 
- ksets do not need a name if they are not registered. 
- Note however that locking must be done manually in these cases.