How to distinguish between full high-water mark and authentication(ZAP) failure when meeting EAGAIN from zmq_msg_send ?

[moyotar]

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Issue description

Environment

• libzmq version (commit hash if unreleased): libczmq-dev 4.2.0-2
• OS: Debian GNU/Linux 10 (buster)

Minimal test code / Steps to reproduce the issue

1. use a socket with ZAP relevant options to connect a server without ZAP options，then try to send several messages and will get EAGAIN.
2. set a small HWM, and try to send messages to get message queue full. We will also get EAGAIN.

What's the actual result? (include assertion message & call stack if applicable)

In Both of the two situation, We will meet EAGAIN from zmq_msg_send.

What's the expected result?

Is there a way to clarify then?