Improve error handling when sending TCP/IP data to DNS server fails (macOS)

[clue]

This changeset improves error handling when sending TCP/IP data to DNS server fails. This is a somewhat expected error condition as exhibited in our test suite. Additionally, there's a somewhat flaky error condition that can only be reproduced in some cases due to a kernel race condition on macOS.

Here's a quick link dump that describes this potential race condition in greater detail in other projects:

• http://erickt.github.io/blog/2014/11/19/adventures-in-debugging-a-potential-osx-kernel-bug/
• https://stackoverflow.com/questions/59914608/error-with-tcp-send-in-python-protocol-wrong-type-for-socket
• write(descriptor:pointer:size:): Protocol wrong type for socket (errno: 41) swift-server/async-http-client#322
• Retry on EPROTOTYPE on socket writes. apple/swift-nio#1706 / Revert "Retry on EPROTOTYPE on socket writes. (#1706)" apple/swift-nio#1707

The gist is that with these changes applied, our test suite now works reliably also on macOS. Additionally, we include more error details to show the particular errno/errstr that caused the error, so this is something that benefits all platforms.

Together with #171, you're looking at ~16h of work. Hunting kernel race conditions is fun.

Builds on top of #171