tcp: Add hystart-plus to cc_newreno and rack.

TCP Hystart draft version -03:
https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-hystartplusplus

Is a new version of hystart that allows one to carefully exit slow start if the RTT
spikes too much. The newer version has a slower-slow-start so to speak that then
kicks in for five round trips. To see if you exited too early, if not into congestion avoidance.
This commit will add that feature to our newreno CC and add the needed bits in rack to
be able to enable it.

Reviewed by: tuexen
Sponsored by: Netflix Inc.
Differential Revision:	https://reviews.freebsd.org/D32373

newlib/libc/sys/rtems/include/netinet/tcp.h

@@ -310,7 +310,7 @@ struct tcphdr {
 #define TCP_RACK_PACING_BETA_ECN 1139	/* Changing the beta for ecn with pacing */
 #define TCP_RACK_TIMER_SLOP 1140	/* Set or get the timer slop used */
 #define TCP_RACK_DSACK_OPT 1141		/* How do we setup rack timer DSACK options bit 1/2 */
-
+#define TCP_RACK_ENABLE_HYSTART 1142	/* Do we allow hystart in the CC modules */
 /* Start of reserved space for third-party user-settable options. */
 #define	TCP_VENDOR	SO_VENDOR