add SO_CNX_ADVICE to sys/socket.h, new in linux v4.6

new socket option so application can give advice about routing
path quality of connected udp sockets, added in linux commit
a87cb3e48ee86d29868d3f59cfb9ce1a8fa63314

include/sys/socket.h

@@ -222,6 +222,7 @@ struct linger
 #define SO_DETACH_BPF           SO_DETACH_FILTER
 #define SO_ATTACH_REUSEPORT_CBPF 51
 #define SO_ATTACH_REUSEPORT_EBPF 52
+#define SO_CNX_ADVICE           53
 
 #ifndef SOL_SOCKET
 #define SOL_SOCKET      1