This is the first step in morphing struct subsystem into something meaningful.

A subsystem is defined simply as a list of kobjects of a certain type, which
is far too generic. A subsystem should be representative of a large entity
of code (i.e. a subsystem of the kernel), not just a simple list. 

This changeset:

- Creates struct kobj_type, a descriptor of the type a kobject is embedded
  in.

- Extracts the fields that are specific to a particular object type from
  struct subsystem and puts them in struct kobj_type, which are
  - the object's release method.
  - the sysfs operations for the object type.
  - the default attributes of the object type.

- Adds ptr to struct kobject to point to its type descriptor. 

- Converts the existing subsystem definitions to define struct kobj_type. 


struct kobj_type's are not registered, as they do not have any explicit
representation in the object hierarchy, nor do they have any fields that
need runtime initialization. 

A kobject's ktype should be set when it is registered, like its subsystem.
Note this obviates the need for defining a struct subsystem when an object
type does not need to be kept in a global list.