Problem: Documentation missing for ZMQ_METADATA socket option

Migrate further tests to unity, and split up into individual test cases

Solution: split test case

Solution: split test case

Solution: split test case

Solution: migrate to unity, and split test cases

Solution: migrate to unity, and split test cases

Solution: migrate to unity, and reduce code duplication

Problem: no support for ZMTP 3.1 application metadata

Solution: add appropriate definitions to build

Solution: migrate to unit and split up test cases

Migrate more tests to unity

Solution: add unity dependencies/flags

Solution: migrate to unity

Solution: split up test case

Solution: migrate to unity

Problem: test assertion failures do not clean up properly

Lets the application set per-connection metadata.
Metadata is specified as "X-key:value" and set using zmq_setsockopt, eg:
zmq_setsockopt (s, ZMQ_METADATA, "X-key:value", 11);

The peer can then obtain the metadata from a received message:
char *data = zmq_msg_gets(msg, "X-key");

Solution: add guidelines to tests/README.md

Solution: provide test_context and manage sockets of test_context via setup and teardown

Migrate further tests (test_hwm, test_router_mandatory) to unity

Solution: removed

Solution: fixed description for building tests, added references to unity and unittests

Solution: migrate to unity

Missing IMPORTED_LOCATION_<CONFIG>property for shared-library import-targets (see zeromq/libzmq#2985)

Fixes for mingw cross-compilation

Solution: remove objects optimisation in library build (similar to #2860)
and set PUBLIC compile definitions on all static builds instead of MSVC
only.

* Fixes issue 2963, ref stream_engine.cpp:981

* Fixes issue 2963, ref socks_connecter.cpp:158

* Fixes issue 2963, ref tcp_listener.cpp:144

* Fixes issue 2963, ref tcp_connecter.cpp:423

* Fixes issue 2963, ref socks_connecter.cpp:436

* Fixes issue 2963, ref tcp_listener.cpp:179

* Fixes issue 2963, ref tcp_listener.cpp:268

* Fixes issue 2963, ref tcp_connecter.cpp:160

Solution: change case of `WinSock2.h Iphlpapi.h Rpc.h` to match the
files on disk. This is only noticeable when cross-compiling from a
case-sensitive system so wouldn't get picked up in MSVC or mingw
builds running on a windows machine.

MSDN uses capitalised versions in prose and lowercase in code examples:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms737629(v=vs.85).aspx

Fixes #2978, the missing library message is a little misleading.

Problem: formatting issues in CI

Solution: commit clang-format-diff

Solution: add pragma to make clang-format ignore the external code

* Problem: Still need to port over more files to VxWorks 6.x

Solution: Port more files to VxWorks 6.x

* Problem: Need to port over remaining files to VxWorks 6.x. Also remove POSIX thread dependency for VxWorks (because of priority inversion problem in POSIX mutexes with VxWorks 6.x processes)

Solution: Port over remaining files to VxWorks 6.x. Also removed POSIX thread dependency for VxWorks

* Problem: Needed to modify TCP, UDP, TIPC classes with #ifdefs to be compatible with VxWorks 6.x.

Solution:  Modify TCP, UDP, TIPC classes with #ifdefs to be compatible with VxWorks 6.x