Solution: add diagnostic output

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: use LIBZMQ_UNUSED where necessary

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