[bhnd] Fix DEFINE_CLASS_(2|3) multiple inheritance support.

This diff updates DEFINE_CLASS_2/_3 to support the specification of class
name separately from the class variable name, bringing them into sync
with their API documentation, as well as the behavior of DEFINE_CLASS_0/_1.

Nothing in the tree currently uses the _2/_3 variants, and I can't
find any references to the API outside of commits to the kobj.h
header itself; given the limitation that currently exists, I'd
be surprised if they've ever been used.

Submitted by: Landon Fuller <landonf@landonf.org>
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D6491