* 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

Fixed issue #2979

* Problem: TIPC availability check is too strict

Solution: at build time only check if the API is available. In the tests
do a first check and a skip if the functionality is not available.

TIPC needs an in-tree but not loaded by default kernel module, tipc.ko
to be loaded, which requires root, so it is unlikely to be available on
any build system by default.
This will allow most distributions to ship with TIPC support built in,
and to avoid tests failure if the module is not there.

* Problem: no Travis tests for TIPC

Solution: mark one job with sudo: required and load the kernel module

* Problem: CMake fails when test returns 77 (skip)

Solution: set property to let it mark the test as skipped as intended

* Tests for different TIPC address types and code cleanup

* Adds tests for binding/connecting with different TIPC address types
  using Unity
* Adds error checking for address type misuse

Revert "Problem: mis-detection of threadsafe_static_init causes test failures"

Problem: mis-detection of threadsafe_static_init causes test failures

Solution: do not rely __cplusplus >= 201103L to detect whether the
compiler supports thread safe static initialisation, but check only
the proper feature preprocessor macro.
GCC introduced it in version 8, and Clang in version 6.

Problem: build broken with gcc-4.7

Solution: initialise variable in options.cpp to dummy value to fix
compiler complaint:

 src/options.cpp: In function
  'int zmq::do_setsockopt_int_as_bool_strict(const void*, size_t, bool*)':
 src/options.cpp:121:5: error: 'value' may be used uninitialized in
  this function [-Werror=maybe-uninitialized]
 src/options.cpp: In function
  'int zmq::do_setsockopt_int_as_bool_relaxed(const void*, size_t, bool*)':
 src/options.cpp:135:31: error: 'value' may be used uninitialized in
  this function [-Werror=maybe-uninitialized]

Problem: code duplication in getsockopt/setsockopt

Solution: extracted common code into do_getsockopt/do_setsockopt functions

Problem: static initialization order fiasco

Problem: CI check for format fails

Solution: add paragraph explaining how to run it to make sure code is
correctly formatted.

Solution: run make clang-format-diff and commit the changes.

Problem: enormous memory increase due to zero copy decoding

The zero copy decoding strategy implemented for 4.2.0 can lead to a large
increase of main memory usage in some cases (I have seen one program go up to
40G from 10G after upgrading from 4.1.4). This commit adds a new option to
contexts, called ZMQ_ZERO_COPY_RECV, which allows one to switch to the old
decoding strategy.

* fix pthread condvar timeouts

This fixes a race condition / hang for threadsafe sockets.
Context: https://github.com/zeromq/czmq/issues/1873#issuecomment-370279244

Solution: use local statics when available in a thread-safe implementation

Problem: unittest_mtrie fails on 32bit *nix

Solution: correctly dereference pointer in test.

Problem: no unit tests for mtrie

Solution: Return an enum from rm instead of a bool, and adapt existing uses

Solution: generalized terms to entries/values

Solution: added test case

Solultion: clarify that notifications for unique subs/unsubs are always
passed, and these option only affect the behaviour for duplicated ones.

Solution: add test_xpub_verbose to cover those APIs

Solution: introduce a type template argument

Solution: added further test cases

Solution: added some TODO comments, improved existing documentation

Solution: added test case, reduced code duplication in tests

Solution: added unit test and assertions

Solution: add test case

Solution: added some test cases for rm

Solution: added test case

Solution: add CODE_COVERAGE_*FLAGS to unit tests build

Fixes #2949

Solution: add some more test cases

Solution: made mtrie generic (to remove complex dependency on pipe_t),
and added some unit tests