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/mkerrors.sh

@@ -232,6 +232,7 @@ struct ltchars {
 #include <linux/if_packet.h>
 #include <linux/if_xdp.h>
 #include <linux/input.h>
+#include <linux/kcm.h>
 #include <linux/kexec.h>
 #include <linux/keyctl.h>
 #include <linux/landlock.h>
@@ -504,6 +505,7 @@ ccflags="$@"
 		$2 ~ /^O?XTABS$/ ||
 		$2 ~ /^TC[IO](ON|OFF)$/ ||
 		$2 ~ /^IN_/ ||
+		$2 ~ /^KCM/ ||
 		$2 ~ /^LANDLOCK_/ ||
 		$2 ~ /^LOCK_(SH|EX|NB|UN)$/ ||
 		$2 ~ /^LO_(KEY|NAME)_SIZE$/ ||