Win32: added WSAPoll() support. WSAPoll() is only available with Windows Vista and newer (and only available during compilation if _WIN32_WINNT >= 0x0600). To make sure the code works with Windows XP, we do not redefine _WIN32_WINNT, but instead load WSAPoll() dynamically if it is not available during compilation. Also, sockets are not guaranteed to be small integers on Windows. So an index array is used instead of NGX_USE_FD_EVENT to map events to connections.