Add an extra space after "/Z7" in CMakeLists.txt.

Both libzmq and libzmq-static are trying to work in precompiled.pch. …

Add an extra space after "/Z7". Else the flag gets joined with the flag following this and clang-cl does not like that.

Fixes https://github.com/zeromq/libzmq/issues/3776.

Code style improvements

Solution: use static_cast instead

Solution: add const/static

Solution: add const

Solution: remove them

Solution: use standard loop constructs and optimize some loops

Solution: consistently use virtual, override and final

Problem: sockets can be created after calling zmq_ctx_shutdown

* Clarify used C++-standard for compilation.

* More than a conforming compiler may be necessary to compile the library. Just refer to the list of known "good" platforms below.

Solution: fix handling of _starting and _terminate flags

Add tests for this situation.

Clarify documentation of zmq_ctx_shutdown and zmq_socket.

Fixes #3792

EAGAIN: Refer to ZMQ_RCVTIMEO when receiving in blocking-mode

Some minor documentation fixes

Replace strtok with strtok_r

Solution: replace tabs with spaces

Solution: added relicence agreement

Solution: Replaced calls to strtok with strtok_r (strtok_s for windows)
in ws_engine.cpp

Problem: WS build option is independent of DRAFT

Solution: make the default depend on the value of the DRAFT build flag

Solution: add them

Problem: NEWS out of date with latest changes

Solution: update it

Documentation fix regarding thread-safety of zmq_poll/zmq_poller.txt.

See https://github.com/zeromq/libzmq/issues/3778.

Fix websocket without path with test

websocket urls without a path caused crash!

problem: ws_engine don't support WS RFC ping-pong and close

Solution: when peer send a close msg, close the connection

Solution: implement both PONG response and producing pings