Make ersatz_socketpair work on IPv6-only systems:
Some systems don't have IPv4 localhost (some IPv6-only systems): ersatz_socketpair currently uses AF_INET. If that fails with EPROTONOSUPPORT (or WSAEPROTONOSUPPORT on Windows), let's have it try AF_INET6.
Some systems don't even have IPv6 localhost (some FreeBSD jails). Instead, they have a non-127/8, non-::1 IP address used as the default: ersatz_socketpair currently uses INADDR_LOOPBACK. On IPv6, it should use ::1 (or a constant or function returning it). If that fails with E(address not found) (or WSAE(address not found) on Windows), that's actually what we want - we don't want to accidentally expose a socketpair on a routable address.
We should also remove the check in the ersatz_socketpair unit test that bails out on EPROTONOSUPPORT.
Patch on a version before 22dba27d8dd5 (23 Nov 2004) / svn:r2943.