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]

* bug fix: AIX pollset error

buf fix:  AIX only, pollset 'rm_fd' set fd_table to null first then set pe->fd to retired_fd

* remove extra spaces

remove extra spaces

Problem: Windows does not support IPV6_TCLASS

Solution: don't use it on Windows.
There is a Windows-specific API for ToS, even IPv4 does not work and
it's just a silent no-op on that platform.

Problem: select broken

Solution: initialise class variables in the same order they are
declared

Solution: fix acinclude.m4 snippet that checks if select is
available to stop it erroring out.

Add a single sentence explaining what a context is

only compare pollitem.fd if pollitem.socket is NULL

item.fd should be ignored if item.socket is defined.

Failing to check .socket could cause false-matches, raising EINVAL in zmq_poller_modify_fd.

Problem: size of zmq_msg_t is not known to FFI wrappers

Solution: mention it in zmq_ctx_get manpage

Solution: mention it as it's a new public API (still draft until release)

Solution: add a ZMQ_MSG_T_SIZE context read-only option so that
wrappers can call zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) to get the
size at runtime.

Document terminating NUL behavior in API more explicitly

This addresses #2169.

Problem: inconsistent indentation and tabs in code

Problem: multicast is spelled incorrectly in udp_address class

Solution: Fix typo of 'mutlicast' -> 'multicast'

Fix htons copy pasta

Problem: IPV6_TCLASS setsockopt fails on IPv4 socket