SwDoc::CopyMasterHeader/Footer(): this could result in sharing the
first-page header/footer with the left-page (!) when un-sharing
via the dialog; the reason is that what actually happens here is that
the left-page header/footer was never changed but the master one
was copied in SwDocStyleSheet::SetItemSet(), so it sort of worked
by accident before the first-page header/footer was added...

Change-Id: Ia24df6ad59cda484559f2ca48ecaa7563878120b