Problem: FD_SETSIZE 1024 is too restrictive under Windows

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.

Fixes #1165