Peter Lamberg filed bug report #2015126: "poll gives WSAEINVAL when POLLPRI

is set in fdset.events" (http://curl.haxx.se/bug/view.cgi?id=2015126) which
exactly pinpointed the problem only triggered on Windows Vista, provided
reference to docs and also a fix. There is much work behind Peter Lamberg's
excellent bug report. Thank You!