Allow preseeding extra options for the local sources.list lines

This patch allows a preseed to specify extra options for the sources.list lines. The concrete use case triggering this change was a local requirement to add check-valid-until=false on a local repository: setting the URL of the repo to "[check-valid-until=false] http://…" was a hack that only worked as long as it didn't require a specific key; when a key was specified, the line ended up like "deb [signed-with=…] [check-valid-until=false] https://…", which was invalid. This patch provides a cleaner way to fullfill that need, without the hackish solution.