SSL shutdown/cancel returns stream_truncated instead of canceled

[sgerbino]

Summary

Stopping an SSL read task returns cond::stream_truncated instead of
cond::canceled, breaking cancellation feedback.

Detail

Stopping an SSL read task returns cond::stream_truncated instead of
cond::canceled — the one place the "operation was canceled" feedback is not
delivered. The mapping to stream_truncated is too aggressive.

Acceptance criteria

• Stopping an SSL read task returns cond::canceled rather than
  cond::stream_truncated.

References

normalize_openssl_shutdown_read_error in openssl_stream.cpp.

Raised by Andrzej Krzemienski; Steve Gerbino agreed the mapping is too
aggressive and is actively triaging. Source: Boost ML "[capy] error codes" +
Slack.