evutil_ersatz_socketpair: use AF_INET6 if AF_INET bind fails
Apparently there are systems where you can bind to ::1, but you can't bind to 127.0.0.1. On these systems, evutil_ersatz_socketpair doesn't work right -- it only falls back to IPv6 when it fails to create an AF_INET socket.
I'm calling this a very low priority bug, since evutil_ersatz_socketpair is only used on Windows, and (as far as I know) binding 127.0.0.1 should always work on Windows.