Solution: increased to 4096 by default for all MSVC builds, for MinGW,
and for CMake.
Note: this is a speculative change, it needs confirmation before we
can keep it. Particularly, there is some doubt that changing this in
libzmq will affect upstream applications using libzmq.dll.
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.
