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

Solution: added test & some improvements of test utils

Solution: emit events as expected by tests, and refuse connections when
ZAP is required but no handler started

Addresses #2711 partially

Solution: added test case for NULL/PLAIN/CURVE

Solution: renamed routing_id fields in pipe_t, renamed ZMQ_CONNECT_RID to ZMQ_CONNECT_ROUTING_ID

Problem: test_zap_unsuccessful_status_500 and test_curve_security_with_bogus_client_credentials sometimes fail, particulary on slow/valgrind runs

Solutio: relax test assertion

Problem: no tests for client-side events for successful handshake and authentication failure in handshake

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

Solution: redesign ZMQ_HANDSHAKE_FAILED_* events, introduce new class of ZMQ_HANDSHAKE_FAILED_AUTH events

Solution: added ZAP tests for PLAIN

Solution: added ZAP tests for NULL

Solution: Move ZAP tests to own file test_security_zap.cpp, move common code to testutil_security.hpp