debian: Use a custom systemd service file

Use a custom systemd service file, until the changes are applied
upstream (see balabit/syslog-ng#316). The changes are that
StandardOutput and StandardError are now redirected to the
Journal (instead of null), and syslog.socket is not pulled in anymore.

Not pulling syslog.socket in fixes balabit/syslog-ng#314.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>