`isrecoverable` check for DNSErrors seems ineffective and needing correction

[tanmaykm]

There are some isrecoverable checks in the HTTP client retry layer here. The check that makes Sockets.DNSError due to EAI_AGAIN non-recoverable seems surprising, and is probably broken.

It checks the error code to match the declared const EAI_AGAIN = 2. However the codes can only be one of these, and the closest matching seems to be UV_EAI_AGAIN which has a value of -3001

julia> Base.UV_EAI_AGAIN
-3001

Which implies that the isrecoverable check for EAI_AGAIN DNS errors are in-effective now, and HTTP client considers all kinds of DNS errors recoverable. Unless there are any complains regarding this behavior, we could probably remove the specific EAI_AGAIN check then?

Or should HTTP client rather not be retrying on certain DNS errors? From the list of possible DNS errors actually only EAI_AGAIN seems recoverable. So should the check be corrected to check against UV_EAI_AGAIN and make that recoverable, and all others non-recoverable?