systemd: remove syslog.socket references from service file

Syslog-ng reads the messages directly from journal on systems with systemd, so
we don't need these settings anymore.
systemd: use Type=notify in service file
Merge pull request #316 from ihrwein/f/systemd-service-file-fix

systemd: remove syslog.socket references from service file
riemann: Do not crash without attributes() set

When using a riemann destination, do not crash when no attributes are
set.

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