Solution: fixed call of select, and initialization of poll_t members

Problem: no test framework, no unit tests

* Problem: build failure with select as polling mechanism

Solution: cast mailbox_handle argument to (poller_t::handle_t) like in
the reaper thread class.

* Problem: build failure due to INT_MAX use without include

Solution: include limits and climits in src/select.cpp where INT_MAX is
used

* Problem: build failure due to unused variable in select.cpp

Solution: move the declaration of int rc inside the ifdef block where
it is actually used

* Problem: reference to wrong variable in select.cpp breaks build

Solution: fix it

* Problem: family_entry_t constructor has no body, build fails on *nix

Solution: add empty inline function in the struct

* Problem: no test coverage for poll and select

Solution: add Travis jobs for them on Linux

* Problem: Travis jobs cannot run in container infra

Solution: set sudo: false as it is not required anymore

Solution: use _snprintf when snprintf is not available

Solution: add it

Solution: add them to Makefile.am, linking to the static libzmq.a
library and its dependencies

Solution: add a noinst convenience static library and use it with the
tests that require it

Solution: add started flag

Solution: set up initial unit tests

Solution: extract thread_ctx_t from ctx_t

Solution: added MIT license file and github source info

Solution: use unity in test_security_curve

Solution: rewrite test_socket_null to use unity

Solution: add unity test framework, and integrate into cmake build

Fixed a deadlock and an assertion failure in poll_t

Solution: use wsa_assert instead of errno_assert

pollset entry

Solution: clean up retired entries before poll

Solution: fixed behaviour in corner cases

Problem: make format-check not implemented with autotools

Problem: DRAFT build broken with GCC 4.7

Solution: port implementation from zproject

Solution: change variable type in test_timers to match public API

Add support for ZMQ_XPUB_NODROP on ZMQ_RADIO sockets

Add support for SIO_LOOPBACK_FAST_PATH on Windows

Solves issue #2927

Solution: add socket option

Solution: activate SIO_LOOPBACK_FAST_PATH if available

See #2899

Problem: assertion failure in poller_base.cpp:42

execution

Solution: relax test assertion to what can be guaranteed

Solution: relaxed test assertions, based on the actual time passed,
instead of assuming that this equals to the time slept

load

Solution: add assertions, partially in debug build only, improved naming

* Problem: race conditions for options.linger

Solution: make options.linger atomic

* Problem: std::atomic not used on Visual C++ although it is available

Solution: change conditional compilation to recognize _MSC_VER

Add windows poll build on Appveyor

Problem: duplicated code, redundant member handle_valid, asymmetry between tcp_connecter and tcp_listener

Problem: code duplication and unnecessary nesting around ZMQ_THREAD_SAFE querying

querying

Solution: remove code duplication and rearrange conditions

Problem: code style issues in mechanism_t and socket_base_t

Problem: several fields are non-const without need

 Problem: unreachable code around socket_base_t::add/remove_signaler