unix: add Linux Kernel Connection Multiplexor types and constants

For details on KCM, see:
https://www.kernel.org/doc/Documentation/networking/kcm.txt.

Change-Id: I14ebf0aaad17f5a9078f027bd50e050c69b08981
Reviewed-on: https://go-review.googlesource.com/c/sys/+/348449
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/types.go

@@ -108,6 +108,7 @@ struct termios2 {
 #include <linux/if_pppox.h>
 #include <linux/if_xdp.h>
 #include <linux/ipc.h>
+#include <linux/kcm.h>
 #include <linux/keyctl.h>
 #include <linux/landlock.h>
 #include <linux/loop.h>
@@ -4043,3 +4044,11 @@ const (
 	IFLA_CAN_DATA_BITRATE_CONST   = C.IFLA_CAN_DATA_BITRATE_CONST
 	IFLA_CAN_BITRATE_MAX          = C.IFLA_CAN_BITRATE_MAX
 )
+
+// Kernel connection multiplexor
+
+type KCMAttach C.struct_kcm_attach
+
+type KCMUnattach C.struct_kcm_unattach
+
+type KCMClone C.struct_kcm_clone