Refactor varobj_update interface.

	Refactor varobj_update interface.
        * varobj.c (varobj_update): Report changes as vector.  Also
        return not just a list of varobj, but a list of special structures
        that tell what exactly has changed.
        * varobj.h (enum varobj_update_error): Rename to
        varobj_scope_status.
        (struct varobj_update_result_t): New.
        (varobj_update): Adjust prototype.
        * mi/mi-cmd-var.c: Adjust for changes.