bnc#893791 XLS export: external sheet references on Linux/OSX

The commit solves two problems.
1. Make sure we save absolute paths on Linux/OSX
2. Make sure we don't save invalid XLS (VirtualPath longer than 255 chars)
The first problem has always been there, so after a few load/save cycles
an XLS with reference to other XLS on a Unix-like file system became
invalid, and only LibreOffice could open it, Excel could not. These
broken XLS files can be repaired by saving after this patch, however,
the original reference will remain broken.

(cherry picked from commit 3f5251675eeeeae56ea282fdeb09dbc53ce4aae6)

Conflicts:
	sc/qa/unit/subsequent_export-test.cxx

Change-Id: I8f575acb1d560d539c1da61a1afdaac3f0c13977
Reviewed-on: https://gerrit.libreoffice.org/12099
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>