problem: appveyor is very slow

Solution: move to github actions

Problem: out-of-bounds array access in socket_poller::check_events.

Problem: poller item lookup can be simplified

Solution: Extract generic find function

problem: checking the wrong socket when sending hello msg

Problem: Spinning on atomics can cause hangs

Fix coverage job and appveyor job

Solution: Add a sleep in the loop. Some versions of
valgrind may hang when spinning on atomic variables.

Solution: bump OS to Xenial for newer python libraries

Solution: don't run it

problem: router doesn't know when peer disconnected 

typo fix root README

Problem: No support to query poller size

Solution: Add zmq_poller_size that queries the number
of objects registered, allowing safer usages of poller
to avoid livelock situations.

ZMQ_ROUTER_NOTIFY doesn't have a context and doesn't play nice with protocols. with ZMQ_DISCONNECT_MSG we can set it to a protocol message, like DISCONNECT in majordomo. Router will send it when a peer is disconnected. Another advantage of ZMQ_DISCONNECT_MSG is that it also works on inproc.

Together with ZMQ_HEARTBEAT it allows to build very reliable protocols, and much simpler as well.

problem: ZMQ_HEARTBEAT is not useful without sending an hello message

When using ZMQ_HEARTBEAT one still needs to implement application-level heartbeat in order to know when to send a hello message.
For example, with the majordomo protocol, the worker needs to send a READY message when connecting to a broker. If the connection to the broker drops, and the heartbeat recognizes it the worker won't know about it and won't send the READY msg.
To solve that, the majordomo worker still has to implement heartbeat. With this new option, whenever the connection drops and reconnects the hello message will be sent, greatly simplify the majordomo protocol, as now READY and HEARTBEAT can be handled by zeromq.

* Allow CMAKE to generate ws and wss transports
I guess there is little use of just ws transport, so by default
GnuTLS (and libsodium) are enabled

* cmake libzmq including wss transport (ubuntu 19.10 and ubuntu 19.10 + wsl 1.0)
test_security_fails (libsodium assert !?)

* updated relicense

* make external libs gnutls nss sodium optional

* #ifdef WSS classes and functions, build test*ws* only if correct libs are included, warning if libs not present

* make libsodium optional

* cmake fix tests TIPC transport

* clang-format pointed out a wrongly placed #ifdef

* GnuTLS before 3.6.7 is not safe

* msvc doesn't agree with strlen in array declaration, test_socks now at least compiles on windows

* windows: libsodium build fails, missing include dirs set by env var

* ws transport test only works when GnuTLS is found

* Fixed condition to use NSS / built in SHA1, so that test_ws_transport should now pass, also when GnuTLS is not found

Problem: poller sleeps forever if no events

Problem: poller item fd uninitialized

Solution: Fail with error if no events are active and timeout is infinite

Solution: Set to zero for socket events

* Problem: boilerplate when init msg from data copy

Solution: Add zmq_msg_init_buffer to construct
a message by copying memory from buffer.

Problem: Strange zmq_poller doc formatting

Solution: Add newlines and extra asterisks

Problem: zmq_send doc does not match declaration

Solution: add const to pointer type

Problem: ZMQ_THREAD_SAFE is not bool

Solution: Change documentation value type to int

Fix gitignore tracking file list

Solution: Fix .gitignore's exceptions for sources in tools/

Install CMake config to LIBDIR by default, to correctly support multiarch

Signed-off-by: FeRD (Frank Dana) <ferdnyc@gmail.com>

Solution: Install CMake config in arch-dependent LIBDIR/cmake

Using "share/cmake/${PROJECT_NAME}" as DESTINATION for installing
ZeroMQConfig.cmake et al works for arch-independent configs, but
is wrong for multiarch. The configs for each version of the
library should be stored below the arch-dependent LIBDIR, using
the GNUInstallDirs ${CMAKE_INSTALL_LIBDIR} variable.
Signed-off-by: FeRD (Frank Dana) <ferdnyc@gmail.com>

Refactor protocol literals into constants