Add "Happy Eyeballs" for IPv4/IPv6.

This patch invokes two socket connect()s nearly simultaneously, and
the socket that is first connected "wins" and is subsequently used for
the connection. The other is terminated.

There is a very slight IPv4 preference, in that if both sockets connect
simultaneously IPv4 is checked first and thus will win.

lib/multi.c

@@ -621,17 +621,26 @@ static int waitconnect_getsock(struct connectdata *conn,
                                curl_socket_t *sock,
                                int numsocks)
 {
+  int i;
+  int s=0;
+  int rc=0;
+
   if(!numsocks)
     return GETSOCK_BLANK;
 
-  sock[0] = conn->sock[FIRSTSOCKET];
+  for(i=0; i<2; i++) {
+    if(conn->tempsock[i] != CURL_SOCKET_BAD) {
+      sock[s] = conn->tempsock[i];
+      rc |= GETSOCK_WRITESOCK(s++);
+    }
+  }
 
   /* when we've sent a CONNECT to a proxy, we should rather wait for the
      socket to become readable to be able to get the response headers */
   if(conn->tunnel_state[FIRSTSOCKET] == TUNNEL_CONNECT)
     return GETSOCK_READSOCK(0);
 
-  return GETSOCK_WRITESOCK(0);
+  return rc;
 }
 
 static int domore_getsock(struct connectdata *conn,