Support regression curve name in old binary XLS XP/2k/2k3 format.

Change-Id: Ie6f2fd3cce70e4789805e788846d3beb64620a4b
Reviewed-on: https://gerrit.libreoffice.org/6876
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>