F/add systemd source

By using the constant, I meant using `SD_LISTEN_FDS_START` without wrapping it in a function. Using the value of it would not be good, indeed, but using it by name is fine. It will not be changing anytime soon, and I can't see us using it from more places than this file.

We can wrap it away into a function at a later time, when there is a good reason to do that. Right now, there isn't. We don't wrap the various socket options, or the various OpenSSL constants, either.