Remove duplicate poller decision making

The decision about the poller mechanism to use (select, poll, ...)
was done twice: once by the build system and once by the code in
poller.hpp. As the build-system can actually detect the mechanisms
available, prefer that result to the hard coded defaults in
poller.hpp.

At the same time, remove the duplicate detection of select() vs.
poll()-variant from proxy.cpp, signaler.cpp and zmq.cpp.

This patch has not been tested on many build platforms: especially
the cmake build needs testing / patching. For the other builds,
hard code the result as these these are all Windows platforms.

Added temporary CURVE debugging support

- just prints server-side failures to console
- can be improved over time, e.g. enabled at build time or
  sent to inproc debug channel

Clarify configure messages for --with-poller=...

Update .gitignore to exclude *.exe under Cygwin

Rename ZMQ_MAX_SOCKETS_MAX to ZMQ_SOCKET_LIMIT

As per suggestion by Pieter Hintjens. Also update wording in
zmq_ctx_set manual a bit.

Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()

Fix a strict-aliasing with type-punning

Reverse zmq_proxy_chain and zmq_proxy_hook

This reverts commit 9ae6a91f.

This reverts commit bc7441f5.

Fixes #880

The new options allows querying the maximum allowed number of sockets.
This is system dependent and cannot be encoded in the include file as a
preprocessor macro: for ZMQ_USE_SELECT, this depends on the FD_SETSIZE
macro at time of library compilation, not at time of include file use.

fix libzmq.pc generation under cmake

fix autotools out of tree build link issue on linux

Fix hang on terminate when inproc is connected but never bound.

added srcdir to --version-script=$(srcdir)/libzmq.vers

The ${libdir} was getting replaced/removed by configure_file() making pkg-config give bad flags: -L -lzmq
My fix was to add @ONLY to configure_file() so ${} style pkg-config substitutions are left alone.
In addition, I put the other typical ${} substitutions back into the libzmq.pc, since its now safe.

Revert "Fix potential memory leak"

This reverts commit 50d34e56.

Fix identity handling for inproc transport

Fixes #872

Added ZMQ_BUILD_TESTS to CMakeLists.txt to allow turning off tests. 

Added ZMQ_BUILD_TESTS to CMakeLists.txt to allow turning off tests. Useful when building ZMQ as a git submodule.

Fix potential segfault/memleak

Delete the socket if the initialization part of the socket creation
fails.

Call the allocation assertion macro before dereferencing the socket
pointer.

Fixed issue #868

Fixed issue #865 - validation on ZMQ_TCP_KEEPALIVE