add a go.mod file

[mvdan]

I realise that this library is still usable without being a module directly, but I imagine there's no reason to avoid adding a go.mod file either?

One good reason to add a go.mod file is that one can use commands like go build, go list, and so on. Another is UX: it's not entirely clear that I'm meant to import this as inet.af/netstack rather than github.com/inetaf/netstack if I don't see a go.mod saying as much.

[mvdan]

Huh, the README even mentions and links to a go.mod file in this repository, but it's a 404.

[mvdan]

Seems particularly necessary too, given that this library requires three other modules.

Looks like the file was added/modified in 0407d1a. It hasn't been deleted since, that I can see in the git log. Perhaps one of the git merges somehow dropped the old master history, and entirely replaced it with an upstream history lacking the go.mod file?

[bradfitz]

Whoops. I have a huge complicated abomination of a shell script with gnarly git commands to build this repo and I guess I lost a step in there somewhere.

[polynomialspace]

Looks like it was lost in 1c1bcf4, which just happened to be a few days after google/gvisor@d350c95 bumped go.mod to 1.17.