Solution: unify the two APIs, as they are both still in DRAFT state and
thus can be changed.

Solution: fix them

Solution: use unlikely to optimize for the most common case

Solution: remove them, only public headers have to be ifdef'd

Solution: list it

 Support XPub socket send last value caching to last subscription pipe with ZMQ_XPUB_MANUAL_LAST_VALUE. (#3511)

* Add ZMQ_XPUB_MANUAL_LAST_VALUE

* Surpport xpub send last value caching to one pipe with ZMQ_XPUB_MANUAL_LAST_VALUE

* Add test_xpubub_manual_last_value

* Add relicense and doc

Problem: Potentially unitialized variable in #3508

Solution: Create a unique endpoint for each fcn call.

Solution: Initialized it

Problem: Socket monitoring only allows ZMQ_PAIR

Solution: Allow ZMQ_PUB and ZMQ_PUSH sockets types for the monitoring.
This way someone could create a ZMQ_PULL socket connected to multiple
monitoring sockets at the same time.

Problem: PLAIN HELLO message incorrectly uses WELCOME literal

Solution: change to HELLO

Problem: signature of zmq_poller_fd does is incompatible with regular…

Solution: change return type to int (again) and return fd via an output parameter

Problem: syntax error on Windows related to socket descriptor type

Solution: use proper fd_t type, and also define and use a zmq_fd_t in the API

Problem: generated ZeroMQConfig.cmake does not work when only static …

Solution: check for availability of shared/static libzmq targets

Some cleanup in CMakeLists.txt

Solution: use check_cxx_symbol_exists resp. check_type_size instead

Solution: Group include directives together at the beginning of the CMakeLists.txt file

Solution: remove it

Solution: fix spelling

Solution: remove redundant debug output

Fix a typo in root CMakeLists.txt

Problem: build broken on Fedora 30

Solution: use AC_COMPILE_IFELSE instead of AC_CHECK_FUNCS to check for
strnlen, as the latter does not work anymore.

Problem: Missing doc & unit tests for zmq_poller_fd

Solution: Add some and document it.

Solution: Add doc & unit tests

Problem: zmq_socket_monitor example did not include HANDSHAKE_SUCCEEDED event causing assert failure

 Problem: Cannot get thread safe socket fd 

Solution: add one

Solution: Add a method to get the zmq_poller's signaler fd. Then we can
associate a poller instance with every thread safe socket and use its
fd.