Problem: strict ZAP protocol adherence is backward incompatible

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

src/options.hpp

@@ -242,6 +242,9 @@ namespace zmq
 
         // Device to bind the underlying socket to, eg. VRF or interface
         std::string bound_device;
+
+        //  Enforce a non-empty ZAP domain requirement for PLAIN auth
+        bool zap_enforce_domain;
     };
 }