* Extracted connect_vanilla_socket function

* Problem: no tests for ZMTP-CURVE protocol errors

Solution: added two test cases with erroneous HELLO commands

* Problem: insufficient tests for ZMTP-CURVE protocol errors

Solution: added two test cases with erroneous HELLO command version

* Problem: test HELLO message is invalid apart from deliberate errors

Solution: create cryptographically correct HELLO message
add tweetnacl.c to test_security_curve

* Problem: nonce is incorrect, build fails with GCC

Solution: use correct non prefix

* Problem: make builds are failing

Solution: transfer CMake changes to (auto)make files

* Problem: nonce is incorrect, build fails with GCC

Solution: use correct non prefix

* Problem: make builds are failing

Solution: transfer CMake changes to (auto)make files

* Problem: no test with INITIATE command with invalid length

Solution: added test case

* Problem: code duplication between test_security_curve.cpp and curve_client.cpp

Solution: extracted parts of zmq::curve_client_t::produce_hello into reusable function

* Problem: code duplication between test_security_curve.cpp and curve_client.cpp

Solution: extracted further parts of zmq::curve_client_t into reusable functions
added missing file

* Problem: mechanism_t::add_property can be declared static

Solution: declare mechanism_t::add_property static

* Problem: intermediate crypto data needs to be passed between static function calls to curve_client_tools_t

Solution: add non-static member functions

* Problem: msg_t instance may be closed twice

Solution: remove offending close

* Problem: prepare_hello uses static curve_client_tools_t::produce_hello

Solution: Use non-static curve_client_tools_t::produce_hello

* Problem: no test with invalid command name where INITIATE command is expected

Solution: added test case

* Problem: make builds are failing due to curve_client_tools.hpp not being found

Solution: add curve_client_tools.hpp to list of source files

* Problem: wrong initializer order in zmq::curve_client_t

Solution: reorder

* Problem: under non-Windows systems, test fails because random_open was not called

Solution: call random_open/random_close within test

* Problem: conflict between custom function htonll and macro definition on Darwin

Solution: define htonll function only if not defined as a macro

* Problem: nullptr not defined on all platforms

Solution: replace nullptr by NULL

* Problem: libsodium builds not working

Solution: adapt compile and link file sets for libsodium builds

* Problem: Makefile.am broken

Solution: Fix syntax

* Problem: no tests for garbage encrypted cookie or content in INITIATE

Solution: added test cases

* Problem: test cases accidentally excluded from build

Solution: remove #if/#endif

* Solution: some error cases are unreachable

Problem: for the time being, added some comments without changing the code

* Added comments on hard-to-test cases