tls-endpoint: Listen on systemd socket

[Julow]

Add support to albatross-tls-endpoint for listening on a systemd socket. This is useful to let systemd open priviledge ports safely.

The '--port' option is kept for compatibility and could also be added to the other commands thanks to a reusable cmdliner term.

[hannesm]

I think this is fine, but would like to hear from @reynir before merging.

[Julow]

The systemd service for the TLS endpoint needs to pass the public/private keys as argument. I'm not sure how to package that? The nixos module is an example :)

[reynir]

I added systemd examples for albatross-tls-endpoint. If you could take a look and see if it looks reasonable I'll merge this. Sorry for the long process.

[Julow]

Thanks! No need to make an example that runs immediately indeed.