unix: implement Linux AF_VSOCK sockets

Adds the required constants and new SockaddrVM type for working
with Linux VM sockets.  For more information, see:
https://vmsplice.net/~stefan/stefanha-kvm-forum-2015.pdf.

Fixes golang/go#19434

Change-Id: I9aefb35441b2c7dc412bdfb806ebe08d5c20f6b1
Reviewed-on: https://go-review.googlesource.com/37937
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkerrors.sh

@@ -132,6 +132,7 @@ includes_Linux='
 #include <linux/icmpv6.h>
 #include <linux/serial.h>
 #include <linux/can.h>
+#include <linux/vm_sockets.h>
 #include <net/route.h>
 #include <asm/termbits.h>
 
@@ -352,6 +353,7 @@ ccflags="$@"
 		$2 ~ /^ALG_/ ||
 		$2 ~ /^GRND_/ ||
 		$2 ~ /^SPLICE_/ ||
+		$2 ~ /^(VM|VMADDR)_/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||
 		$2 ~ /^BLK/ {printf("\t%s = C.%s\n", $2, $2)}