ss: report ecnseen

Support ECNSEEN reporting in ss command.

ESTAB      0      0           10.170.73.123:4900
10.170.73.125:51001    uid:501 ino:385994 sk:f31e5f00
         mem:(r0,w0,f0,t0) ts sack ecn ecnseen bic wscale:8,8 rto:210
rtt:18.75/15 ato:40 cwnd:10 send 69.9Mbps rcv_space:32768

"ecn" means TCP session negociated ECN capability (TCP layer) at setup
time

"ecnseen" at least one frame with ECT(0) or ECT(1) or ECN (IP layer) was
received from peer.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

include/netinet/tcp.h

@@ -172,6 +172,7 @@ enum
 # define TCPI_OPT_SACK		2
 # define TCPI_OPT_WSCALE	4
 # define TCPI_OPT_ECN		8
+# define TCPI_OPT_ECN_SEEN	16
 
 /* Values for tcpi_state.  */
 enum tcp_ca_state

misc/ss.c

@@ -1363,6 +1363,8 @@ static void tcp_show_info(const struct nlmsghdr *nlh, struct inet_diag_msg *r)
 				printf(" sack");
 			if (info->tcpi_options & TCPI_OPT_ECN)
 				printf(" ecn");
+			if (info->tcpi_options & TCPI_OPT_ECN_SEEN)
+				printf(" ecnseen");
 		}
 
 		if (tb[INET_DIAG_CONG])