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.

src/event/ngx_event_connect.c

@@ -388,7 +388,16 @@ ngx_event_connect_set_transparent(ngx_peer_connection_t *pc, ngx_socket_t s)
             return NGX_ERROR;
         }
 
+#else
+
+        ngx_log_error(NGX_LOG_ALERT, pc->log, 0,
+                      "could not enable transparent proxying for IPv6 "
+                      "on this platform");
+
+        return NGX_ERROR;
+
 #endif
+
         break;
 
 #endif /* NGX_HAVE_INET6 */