systemd won't start tinc while using the upstream service file

[letompouce]

Short version: upstream tinc 1.0.28 introduces systemd services files. Because python-dnspython is (right now) not available from stable but only from backports, tinc will also be installed from backports version 1.0.28-1~bpo8+1 instead of stable version 1.0.24-2.1+b1, thus using the upstream systemd services files.

The problem is, tinc won't start from systemd - but works fine when started manually.

Pull #194 has 4473fc7 which ensures tinc is installed from stable.

I have a feeling that upstream tinc is not the giulty here, it might be the way we use systemd that is wrong. This issue is created to track down this behaviour, if/whenever this behaviour happens again.