systemd socket activation for http/https port

[scottlamb]

This is probably the most user-friendly way to bind to privileged ports 80 and 443 on a modern Linux system. (One alternative would be setting up a firewall rule to redirect to 8080/8443 or some such.)

@dolfs found a nice article about it: https://www.darkcoding.net/software/systemd-socket-activation-in-go/. One of the drafts of #15 also sets up the systemd stuff for it. Not mentioned in that article: I think we'd want to use port names so we can distinguish http vs https vs maybe some other future port (rtsp for clients to use? websocket? rpc interface? who knows).

This will be more important after completing #27. Until then, ports 80 and 443 should belong to the proxy server anyway.

[scottlamb]

I'm moving this from the 1.0? milestone to wishlist because (a) for now the recommended setup uses a proxy server, and (b) the install instructions focus on a Docker setup instead of systemd.