ONE: XREF data and private-mode libraries

If XREF ON is set for a library which is used in private mode,
then Xref data are also generated for the modules in the private-mode library that belongs to this library.

This is superfluous.

Reasons:

- Logon to a private-mode library is not possible,
thus List Xref in this library is also not possible.

- In the case of data areas and copycode it can happen
that Xref data of the private-mode library are shown in the 'base' library
(when a data area of the base library is used by a module in the private-mode library)
This is confusing / unhandy for the users.

So, in private-mode libraries no XREF data should be generated.