add support for --target-release for __package_apt

Signed-off-by: Nico Schottelius <nico@freiheit.schottelius.org>
fix #343 - close fds of mkstemp for messages

Signed-off-by: Nico Schottelius <nico@freiheit.schottelius.org>
update changelog

Signed-off-by: Nico Schottelius <nico@freiheit.schottelius.org>
document change

Signed-off-by: Nico Schottelius <nico@freiheit.schottelius.org>