Use sd_notify for better integration with systemd

[ibukanov]

For better integration with systemd it would be nice to use sd_notify calls when available. This way systemd unit file can use type=notify allowing systemd to track properly service lifetime.

[dlundquist]

I don't intend to include this level of systemd integration. While systemd provides a useful dependency based startup well suited for desktop environments, its expanding scope and integration with its specific APIs is is harmful to the larger ecosystem. While I have included multiple code paths for some Linux specific socket features, these specifically extend the functionality or performance of SNIproxy's core functionality. I don't see this systemd notify integration as significantly adding to the core functionally of sniproxy.

Your welcome implement this in your own branch. All the changes could be isolated to sniproxy.c.