unix: add tcp_md5sig on Linux

This structure is used with setsockopt in BGP speaker implementations.

For more information, please see:
https://criticalindirection.com/2015/05/12/tcp_md5sig/.

Fixes golang/go#24830

Change-Id: Ic2137e843ad502dbe9a4227362b63164789c5877
Reviewed-on: https://go-review.googlesource.com/106656
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/linux/types.go

@@ -67,6 +67,7 @@ package unix
 #include <linux/taskstats.h>
 #include <linux/cgroupstats.h>
 #include <linux/genetlink.h>
+#include <linux/socket.h>
 
 // abi/abi.h generated by mkall.go.
 #include "abi/abi.h"
@@ -956,3 +957,9 @@ const (
 	CBitFieldMaskBit62 = C.BITFIELD_MASK_62
 	CBitFieldMaskBit63 = C.BITFIELD_MASK_63
 )
+
+// TCP-MD5 signature.
+
+type SockaddrStorage C.struct_sockaddr_storage
+
+type TCPMD5Sig C.struct_tcp_md5sig