connect: Add connection delay to Happy Eyeballs.

This patch adds a 200ms delay between the first and second address
family socket connection attempts.

It also iterates over IP addresses in the order returned by the
system, meaning most dual-stack systems will try IPv6 first.

Additionally, it refactors the connect code, removing most code that
handled synchronous connects. Since all sockets are now non-blocking,
the logic can be made simpler.

lib/multi.c

@@ -874,6 +874,7 @@ CURLMcode curl_multi_wait(CURLM *multi_handle,
 
   if(nfds) {
     /* wait... */
+    infof(data, "Curl_poll(%d ds, %d ms)\n", nfds, timeout_ms);
     i = Curl_poll(ufds, nfds, timeout_ms);
 
     if(i) {