Bug 35291: Don't allow symlinks for link files in cover image ZIP

Bug 35291: Don't allow symlinks for link files in cover image ZIP

There's no reason to allow symlinks for link files in cover image
ZIP files. Preventing their use prevents someone from uploading
a symlink pointing to an existing file on the Koha server.

Test plan:
0. Apply patch and restart/reload Koha
1. Create a PNG cover image
2. Create a datalink.txt file that contains something like the
following:
29,Untitled.PNG
3. Turn on "LocalCoverImages" system preference
4. Upload via http://localhost:8081/cgi-bin/koha/tools/upload-cover-image.pl
5. Go to http://localhost:8081/cgi-bin/koha/catalogue/detail.pl?biblionumber=29
6. Note the cover image has been uploaded

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>