Problem: name confl from windows.h, draft API decl in pch.

Problem: zmq_poller_destroy can segfault

Problem: msvc builds are misconfigured.

Problem: test send count too high for AppVeyor CI environment

Solution:
Reduce send count so that the test does not timeout on AppVeyor CI
environment and older Windows boxes.

Solution:
- Add checks for **poller_p_ to ensure that we do not segfault when either it
  or the value within it are NULL
- Add tests for the above and increase error state coverage

Solution:
- Add error state coverage for zmq_ctx_term(), zmq_term() and
  zmq_ctx_shutdown(); zmq_ctx_destroy() is already covered since it only
  calls zmq_ctx_term()
- Add coverage for zmq_term()

Problem: scheduling parameters unused on OSX

Solution:
Mark them with LIBZMQ_UNUSED macro as per convention; although in future the
appropriate pthread code should be updated to support thread scheduling
priorities (for Mac OS X, et. al.)