errno ignored for unix getaddrinfo

[jwilm]

When getaddrinfo() fails, the wrapper produces a custom io::Error using gai_strerror(). There's one variant which essentially says just look at errno.

EAI_SYSTEM    system error returned in errno

But libstd doesn't do that which leads to very unhelpful errors.

Io(Error { repr: Custom(Custom { kind: Other, error: StringError("failed to lookup address information: System error") }) }))

It would be great if the error value could be checked for EAI_SYSTEM, provide the strerror() message in the StringError value, and set Kind appropriately.

[alexcrichton]

Sounds reasonable to me! Should hopefully be more tackle-able once rust-lang/libc#402 lands.

[apasel422]

I think this was closed by #36754.