Getaddrinfo raises error on some backends about eio

[haesbaert]

Aye, it returns empty as you expected.
The thing is Unix.getaddrinfo completely ignores errno, it simply checks if it got 0 and then tries to build a list, returning empty if nothing is there.
We should probably do the same, catch all Luv_errors and return an empty list.

[Sudha247]

Should we close this, now that Eio doesn't have the luv backend anymore?

[talex5]

It also affects eio_posix (I've updated the title).

[joprice]

I'm hitting an issue via cohttp-eio with a statically compiled binary using musl, where getaddrinfo fails to resolve dns names. I hit this branch https://github.com/mirage/ocaml-cohttp/blob/3cd08e5a34d38a322055390b86f4b5dd6bdc01e7/cohttp-eio/src/client.ml#L82 with the error "failed to resolve hostname". It seems not relying on the glibc getaddrinfo would be nice for this reason as well.