debian: README.source update

Updated the README.source documentation to better reflect reality.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
debian/tools: New tool: export-dsc.sh

This tool automates the task of exporting debian source packages: a
systemd one, and a sysvinit one.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>