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
Merge pull request #898 from hintjens/master

Added temporary CURVE debugging support
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.
Merge pull request #899 from olafmandel/duplicate_poller_detect

Remove duplicate poller decision making
fix: win32 tcp_keepalive gets set even when option is -1
Merge pull request #900 from pijyoi/master

fix: win32 tcp_keepalive gets set even when option is -1
workaround missing Mstcpip.h in mingw32
Merge pull request #901 from pijyoi/master

workaround missing Mstcpip.h in mingw32