Today it is not possible (AFAICT) to detect if a DNSError if of type EAI_AGAIN, i.e. if it is something temporary that should be retried. This information is available inside addrinfoErrno but when the DNSError is created this information is lost.

This PR fixes this so that the addinfoErrno.Temporary information is added to DNSError as well. With that a user who gets a DNSError can check now is its a temporary error (for errors that resulted from a addrinfoErrno this is EAI_AGAIN).

Change-Id: I64badb2ebd904e41fc2e0755416f7f32560534d8
GitHub-Last-Rev: ced7238a6597039fb23f36f372bd1cf33d60d4a6
GitHub-Pull-Request: golang/go#31676
Reviewed-on: https://go-review.googlesource.com/c/go/+/174557Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>