Problem: contributing guidelines unclear for new devs

Solution: port Github PR template from CZMQ

Solution: Revert "Problem: Duplicated code"

This reverts commit 022eeaf3.

Problem: Duplicated code

Solution: set_curve_key method in options_t struct with little
improvement in switch-case block

* Fixed handling of WSAEWOULDBLOCK to be generic

I don't know what was the intention of this early if statement but
now this is properly evaluated in wsa_error_no function if this is
performance issue I suggest moving evaluating this error code to the
beginning of wsa_error_no.

* Fixed handling of WSAEWOULDBLOCK to be generic

Introduced default pointer to const char * and overrides this as NULL
if function is called by zmq::wsa_error()

* Fixed handling of WSAEWOULDBLOCK to be generic

Introduced default pointer to const char * and overrides this as NULL
if function is called by zmq::wsa_error()

zmq_poll: only compare FD when neither item is a zmq socket

fix bug zmq4.x PUB msg to ZMTP1.0 SUB server

Problem: no mixed FD and zmq socket zmq_poll test

Solution: add one

misuse of 'close'

use 'pollset_destroy' instead of 'close' to release pollset_fd

Problem: check for item.fd causes zloop_poller regression

Solution: fix the check for the socket.
This regression happens when using zloop with zmq_pollitem_it with
only file descriptors registerted through zloop_poller.

"%" -> '%'

Add #include "precompiled.hpp"

Problem: there is no libunwind-dev on Ubuntu 14.04LTS

Solution: depend on libunwind-dev | libunwind8-dev | libunwind7-dev

Build support for VS2017, Windows SDK 10.0.14393.0, toolset v141

Fix #include case for <mstcpip.h>

On native Windows, the file system is case-insensitive, so this this
doesn't matter there.

However, when compiling on a case-sensitive filesystem, such as when
cross-compiling from a Unixoid using mingw, we have to use the case
the file is supplied with, which is all lowercase.

Problem: test_stream_exceeds_buffer fails to build on AIX

Solution: add missing includes

Solution: remove them as they are already in testutil.hpp

Problem: 4.2.0 won't compile on AIX 7.1

Solution: restore inclusion of poll.h if using poll before zmq.h as
it was originally, as AIX redefines the POSIX structures and provides
compatibility macros.
Also add alternative aliases for 32 bit AIX's pollitem struct:
  events -> reqevents
  revents -> rtnevents

avoid crashing in the multi-thread operation for std::vector

1. stl container is not thread safety
2. rm_fd() and loop() end to clear the retired event source will in multi-thread operation
3. may be crashed in reaper thread to delete the items in the std::vector as the source is nullptr

Problem: PR #2236 breaks the build

Solution: Revert "avoid crashing in the multi-thread operation for std::vector"

This reverts commit e1368bda.

avoid crashing in the multi-thread operation for std::vector

1. stl container is not thread safety
2. rm_fd() and loop() end to clear the retired event source will in multi-thread operation
3. may be crashed in reaper thread to delete the items in the std::vector as the source is nullptr

Problem: CMake build uses library version as the ABI version

Solution: set SOVERSION in CMakeLists to match the SONAME generated
by libtool so that there is no mismatch between the output of the
*NIX build systems.

Before:
$ ls -l
total 2696
lrwxrwxrwx 1 luca luca      15 Dec  1 22:36 libzmq.so -> libzmq.so.4.2.0
lrwxrwxrwx 1 luca luca      15 Dec  1 22:36 libzmq.so.4.2.0 -> libzmq.so.4.2.1
-rwxr-xr-x 1 luca luca  906168 Dec  1 22:36 libzmq.so.4.2.1
$ readelf -d libzmq.so.4.2.1 | grep SONAME
 0x000000000000000e (SONAME)    Library soname: [libzmq.so.4.2.0]

After:
$ ls -l
total 2700
lrwxrwxrwx 1 luca luca      15 Dec  1 22:31 libzmq.so -> libzmq.so.5.1.0
-rwxr-xr-x 1 luca luca  906168 Dec  1 22:31 libzmq.so.4.2.1
lrwxrwxrwx 1 luca luca      15 Dec  1 22:31 libzmq.so.5.1.0 -> libzmq.so.4.2.1
$ readelf -d libzmq.so.4.2.1 | grep SONAME
 0x000000000000000e (SONAME)    Library soname: [libzmq.so.5.1.0]