When SYN or SYN/ACK had to be retransmitted RFC5681 requires us to

reduce the initial CWND to one segment.  This reduction got lost
some time ago due to a change in initialization ordering.

Additionally in tcp_timer_rexmt() avoid entering fast recovery when
we're still in TCPS_SYN_SENT state.

MFC after:	2 weeks

sys/netinet/tcp_output.c

@@ -551,10 +551,14 @@ after_sack_rexmit:
 	 * max size segments, or at least 50% of the maximum possible
 	 * window, then want to send a window update to peer.
 	 * Skip this if the connection is in T/TCP half-open state.
-	 * Don't send pure window updates when the peer has closed
-	 * the connection and won't ever send more data.
+	 *
+	 * Don't send an independent window update if a delayed
+	 * ACK is pending (it will get piggy-backed on it) or the
+	 * remote side already has done a half-close and won't send
+	 * more data.
 	 */
 	if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
+	    !(tp->t_flags & TF_DELACK) &&
 	    !TCPS_HAVERCVDFIN(tp->t_state)) {
 		/*
 		 * "adv" is the amount we can increase the window,