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Problem: NEWS out of date with latest changes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

acinclude.m4: add -latomic to PKGCFG_LIBS_PRIVATE

Add -latomic to PKGCFG_LIBS_PRIVATE so applications linking statically
with libzmq (such czmq) will know that they have to link with -latomic
and the following build failure will be avoided:

  CCLD     src/czmq_selftest
/home/buildroot/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/7.4.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: /home/buildroot/autobuild/run/instance-3/output-1/host/sparc-buildroot-linux-uclibc/sysroot/usr/lib/libzmq.a(src_libzmq_la-ctx.o): in function `zmq::ctx_t::create_socket(int)':
ctx.cpp:(.text+0x1710): undefined reference to `__atomic_fetch_add_4'

Fixes:
 - http://autobuild.buildroot.org/results/4a12f1ede260cd956a0b5ccb4eec6ca8b44cb04fSigned-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

mingw: add missing namespace

Problems: reimplementation of strlcpy, CMake does not use pkg-config for NSS, wrong pc generated by autoconf with GNUTLS

* Update array.hpp

Problem:Calling back on an empty container causes undefined behavior.
Solution:Check with the function empty()

* Create xqcool.mq

Adds relicensing grant

Solution: check correct preprocessor define in test_reconnect_ivl to match the
test invocation

Solution: initialize local variable in all cases to false positive
compiler warning

Solution: add it to one build to exercise all the code paths

Solution: add it to the Requires.private list

Solution: use pkg-config by default and FindPackage only as a
fallback

Solution: use libbsd by default when available, and the internal implementation
only as a fallback, to take advantage of Linux distros maintenance of the
string libraries.

Avoid possible buffers overruns in ws_engine