netinet/tcp: Add TCPOPT, TCPOLEN constants

Programs such as iptables depend on these constants, which can also
be found defined in other libcs.

Since only TCP_* is reserved as part of tcp.h's namespace, we hide
them behind _BSD_SOURCE (and therefore _DEFAULT_SOURCE) to expose
them by default, but keep it standard conforming.

include/netinet/tcp.h

@@ -41,7 +41,20 @@
 #define TCP_CLOSING      11
 
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define TCPOPT_EOL              0
+#define TCPOPT_NOP              1
+#define TCPOPT_MAXSEG           2
+#define TCPOPT_WINDOW           3
+#define TCPOPT_SACK_PERMITTED   4
+#define TCPOPT_SACK             5
+#define TCPOPT_TIMESTAMP        8
+#define TCPOLEN_SACK_PERMITTED  2
+#define TCPOLEN_WINDOW          3
+#define TCPOLEN_MAXSEG          4
+#define TCPOLEN_TIMESTAMP       10
+
 #define SOL_TCP 6
+
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <stdint.h>