mainloop: Add support for systemd notifications

When compiling with systemd support, notify systemd about our status
and readiness via sd_notify().

Also set the service type to Type=notify in the systemd unit file.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
lib/control.c: An EOF on the control channel is not an error

An EOF on the control channel is not an error - that often happens
during normal procedures, when syslog-ng-ctl simply disconnects. So
lower the msg_error() to an msg_notice(), because that's what this
really is.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>