Use systemd notify socket to report process status

It has all sorts of benefits due to the service manager being aware of
the starting/running/stopping state, see systemd.service(5)

On top of that, start-up errors are reported using STATUS= key, so they
will be easier to see in the 'systemctl status' output.

systemd_nonlinux.go

@@ -0,0 +1,14 @@
+//+build !linux
+
+package maddy
+
+type SDStatus string
+
+const (
+	SDReady    = "READY=1"
+	SDStopping = "STOPPING=1"
+)
+
+func systemdStatus(SDStatus, string) {}
+
+func systemdStatusErr(error) {}