Solution: add SECURITY.md with supported branches and maintainers public keys for private communications

Problem: code doesn't follow libzmq naming conventions

Solution: rename types/variables that are inconsistent

Clarify documentation for zmq_unbind/disconnect and new batch options

Solution: restore separate zmq_unbind documentation, adding a note that the implementation
might be shared but the semantics are different and should not be mixed.
This avoids tying the API to the implementation details which might change for some
engines.

Solution: clearly advise users to avoid them in all cases to
avoid unforseen problems.
Also fix markdown syntax, applicable transport types and getter description.

Problem: variable and type names are not descriptive

Problem: {in,out}_batch_size must be configured at compiled time

Solution: use more descriptive names

This commit also improves comments related to some of the changes.

Solution: Added a socket option to configure them at runtime.

Problem: missing heartbeat test for draft sockets

Solution: Add heartbeat tests for CLIENT, SERVER, GATHER and SCATTER
    socket types. RADIO and DISH tests won't be added since heartbeating
    is not yet supported.

Problem: disconnect & unbind doc is misleading

* Problem: radix tree's apply function uses incorrect resizing logic

Solution: rewrite the function using a vector

The buffer is extended once by 256 bytes, which will not be enough if
the node holds data larger than this number.

Make it explicit that the two functions are identical.
Also document the expected disconnect behavior extensively
and illustrate the difference between disconnecting a
connected vs. bound endpoint.

CMake: don't try to link with librt on MinGW

On MinGW, librt will might will found as rt library in MSYS, but fails linking with "cannot find -lrt" error.
As librt is not avaiable on native Windows, we should never try to link with -lrt on MinGW (which is native Windows).

Add tests for SOCKS proxy support

problem: no artifacts for x64 on windows

solution: add x64 builds to appveyor

Solution: don't run tests on tags

This doesn't break anything, as tags was not built until now. Anyway, when building tags we really care about the release of the packages and not tests

Problem: test assertions not reporting the right line number

Solution: pass the line number as part of the testutil functions

problem: appveyor with libsodium change the current directory and bre…

Solution: do git pull without changing the current directoy

problem: no artifact repository for windows

Solution: deploy artifcats to bintray

Problem: formatting errors

Solution: copy it from PR https://github.com/zeromq/libzmq/pull/3537

Solution: apply clang-format-diff

pthread_equal to compare thread ids

I am trying to port ZeroMQ to z/OS USS and I faced several problems. One of them is this function "is_current_thread" trying to compare two thread ids with operator "==". I've changed the code to use the official function "pthread_equal" to compare thread ids and now it's working fine.

Implement SOCKS5 basic authentication

Fix issues with SOCKS5 proxy connection

Two issues where introduced by commit 12c4b55a:
- the proxy connection was done to the target address instead of
the proxy address
- on error the proxy connection status was not reset to unplugged

Problem: Unclear `zmq_msg_send` failure semantics