dareprepro should use a global lock

[geofft]

The build-all infrastructure calls dareprepro with a lock (via flock) so that simultaneous uploads don't race against each other with access to the repository. Since we could have other cases where reprepro is called twice at once -- multiple developers, or a developer and the autodebathenifier, or whatever -- and we now have a dareprepro script, we should move the lock code to that script so it's used everywhere.