build-system: remove use of git-buildpackage, dput. Add pristine-tar.

The previous setup was dependent on the git-buildpackage configuration
to find the resulting tar file, and consequently a bit fragile.

We use pristine-tar instead to save a checksum-identical copy of the
tar file. This will also faciliate "non-native" debian packages, if
desired.

dput again depends on the local configuration, and mainly is a bit too
brave for me to do automatically.