Problem: MS VC++ build broken

Solution: try to detect architecture if building with VC++ and
hardcode pointer size accordingly.
Expressions are not allowed inside declspec intrinsics, which
includes other intrinsics.

Problem: pointer union for zmq_msg_t is a hack

Solution: use compiler's alignment attributes instead which is
clearer and less of a hack.
Pointer alignment violations causing crashes on architectures
such as sparc64 and aarch64.
This also avoid triggering ABI checkers as the change is compatible
even though applications that suffer from the bug should rebuild to
take advantage of the fix.

clock_gettime is now defined in macOS 10.12 SDK

Solution: Rename the custom implementation of clock_gettime for macOS to
alt_clock_gettime and wrap all usage in preprocessor macros to only enable the
alternative version when using macOS <= 10.11.

This issue occurs when targeting macOS 10.11 or earlier but using the 10.12
or newer SDK.

Problem: build API incompatible with 4.1

Solution: keep the new --without-docs option, but also keep the old
--without-documentation with an added deprecation warning.
We can then remove it in the next major release, to leave enough time
for users and maintainers to change it without disruptions.

Problem: 2 connects with same sourceip:port to different destip:port fail

Solution: during a connect with a TCP endpoint if a source address is
passed set the SO_REUSEADDR flag on the socket before the bind system
call.
Add unit test to cover this case for both IPv4 and IPv6.

Update INSTALL to note issues with 'make -j check'

Some of the tests cannot be run in parallel; until the Makefile is fixed, note this in INSTALL.

Problem: Does not build in Visual Studio 2013 / Win32

Solution: #include <netioapi.h> and change some IP_ADAPTER_UNICAST_ADDRESS data types

Add EHOSTUNREACH to errno_to_string

performance enhancement

use clock_gettime if there is no instruction to get cpu tick. It will take about 10% performance enhancement in AIX 7.1.

* - Fixed windows build errors
- Extended monitor lock scope to prevent race-condition between
  process_stop and monitor

* - Fixed windows build errors
- Extended monitor lock scope to prevent race-condition between
  process_stop and monitor

#2158: Add mutex for monitor socket

Fixed issue #2155

Added fallback mechanism for specific socket binding on Windows platform with IPv6 enabled

# Conflicts:
#	builds/msvc/vs2012/libsodium.import.props

Patch for compatibility with Windows subsystem for linux

Problem: Invalid markup in zmq_socket.txt

Solution: Fix it.

Problem: a request

Solution: honor it

doc/zmq_socket.txt: clarify that ROUTER can be blocking

Fix zmq poller poll return

* Doing so was zmq_poller_poll to always return 0.

allow duplicate entries in zmq_poller_poll

This should restore full compatibility with earlier zmq_poll behavior.

It complicates things a little bit, as collisions must be detected, and when collisions are found:

- event masks must be merged
- pollitems, events arrays are no longer co-ordered

Reverts the recent zmq_proxy patch to workaround the lack of repeat-item support in zmq_poll that is now fixed.