Bug#919644: Outdated version of debhelper used for building package

[Julian Dammann]

We investigated the problem further:
The systemd-tmpfiles call in the postinstall script is added by dh_installinit (debhelper).
debhelper Version 12 (sid) and Version 12~bpo9+1 (stretch-backports) already fixed this using basename, as we initially suggested.
However, apparently packages for stretch are not built using the debhelper suite from backports.