Solution: like for other mechanism, do not enforce strict ZAP protocol
adherence unless the specific socket option is enabled with NULL auth.
Add test to exercise this functionality, and fix ZAP test to set the
socket option when it uses NULL auth.
See: https://github.com/zeromq/pyzmq/pull/1152
Solution: add ZMQ_ZAP_ENFORCE_DOMAIN to hide backward incompatible
change and make it disabled by default.
In a future release that breaks API compatibility we can then switch
the default to enabled in order to achieve full RFC compatibility.
Fixes#2762
receive an ERROR (probably because the connection is closed before)
Solution: wait for client-side monitor events before closing the client
socket
Fixes#2705
Solution: added tests for CURVE, add emitting of client-side event in curve_client_t; add ZAP code 300/500 tests for all mechanisms; suppress sending an error message for ZAP code 300