Bug 2734: fix compile errors from CBDATA_CLASS2()

Bug 2734: fix compile errors from CBDATA_CLASS2()

Some compilers don't like the sytax private:;

This makes the starting state explicitly private: instead of assuming its
going to be defined in a private state and dependign on the definer class.

We can cope with not setting the state to private at the end of the macro
as well. It just means the use of this macro must be last, or have an
explicit private/public definition after its use.