gpgtar: Prefer --set-filename over implicit name

* tools/gpgtar-extract.c: Prefer opt.filename over filename
for the directory prefix.

If you would extract from stdin (filename -) and use set-filename
to provide a real filename the "-" would be used for the directory
name. With this change an explicit filename is prefered.

• GnuPG-Bug-Id: T3500
• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>