Comment 4 for bug 1432161

We should probably also add mysql.service next to postgres.

> Second, neither the systemd unit file nor the upstart script had support for the
> DAEMON_OPTS variable from /etc/default/quasselcore. This made it difficult for
> administrators to add extra arguments, which is especially true on systemd since
> the unit file is silently overwritten every time the package is updated.

Using default files is discouraged so we shouldn't add new options, see:
https://wiki.debian.org/systemd/Packaging#overriding_options_and_.2Fetc.2Fdefault_handling

Changes to systemd units aren't overwritten if you do it right.
You want to call: systemctl edit quasselcore.service
And type something like:
[Service]
ExecStart=...