Generate error for unsupported IPv6 transparent proxy. On some platforms (for example, Linux with glibc 2.12-2.25) IPv4 transparent proxying is available, but IPv6 transparent proxying is not. The entire feature is enabled in this case and NGX_HAVE_TRANSPARENT_PROXY macro is set to 1. Previously, an attempt to enable transparency for an IPv6 socket was silently ignored in this case and was usually followed by a bind(2) EADDRNOTAVAIL error (ticket #1487). Now the error is generated for unavailable IPv6 transparent proxy.