Add placeholder constants to reserve a portion of the socket option

name space for use by downstream vendors to add custom options.

MFC after:	2 weeks

sys/netinet/tcp.h

@@ -166,6 +166,9 @@ struct tcphdr {
 #define	TCP_KEEPINTVL	512	/* L,N interval between keepalives */
 #define	TCP_KEEPCNT	1024	/* L,N number of keepalives before close */
 
+/* Start of reserved space for third-party user-settable options. */
+#define	TCP_VENDOR	SO_VENDOR
+
 #define	TCP_CA_NAME_MAX	16	/* max congestion control name length */
 
 #define	TCPI_OPT_TIMESTAMPS	0x01

sys/netinet/udp.h

@@ -50,6 +50,8 @@ struct udphdr {
  */
 #define	UDP_ENCAP			1
 
+/* Start of reserved space for third-party user-settable options. */
+#define	UDP_VENDOR			SO_VENDOR
 
 /*
  * UDP Encapsulation of IPsec Packets options.

sys/sys/socket.h

@@ -142,6 +142,15 @@ typedef	__uid_t		uid_t;
 #define	SO_PROTOTYPE	SO_PROTOCOL	/* alias for SO_PROTOCOL (SunOS name) */
 #endif
 
+/*
+ * Space reserved for new socket options added by third-party vendors.
+ * This range applies to all socket option levels.  New socket options
+ * in FreeBSD should always use an option value less than SO_VENDOR.
+ */
+#if __BSD_VISIBLE
+#define	SO_VENDOR	0x80000000
+#endif
+
 /*
  * Structure used for manipulating linger option.
  */

sys/sys/un.h

@@ -57,6 +57,9 @@ struct sockaddr_un {
 #define	LOCAL_CREDS		2	/* pass credentials to receiver */
 #define	LOCAL_CONNWAIT		4	/* connects block until accepted */
 
+/* Start of reserved space for third-party socket options. */
+#define	LOCAL_VENDOR		SO_VENDOR
+
 #ifndef _KERNEL
 
 /* actual length of an initialized sockaddr_un */