unix: implement AF_TIPC sockets on Linux

Adds initial constants and types to use with SockaddrTIPC. For more
information on TIPC, see: http://tipc.sourceforge.net/.

Because the C struct sockaddr_tipc makes use of a union for its addr
field, I have created a wrapping layer in SockaddrTIPC that uses
an interface to determine which variant is in use.

Tests accompany this change due to the complexity of this code. There
is currently little test coverage in the Sockaddr-related code, but
perhaps this is something that could be picked up as a relatively
straightforward task for a new contributor.

Change-Id: I5033a0685cb7128d4b1a23d18aca71c202d0c0aa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/187960
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

@@ -227,6 +227,7 @@ struct ltchars {
 #include <linux/rtc.h>
 #include <linux/if_xdp.h>
 #include <linux/cryptouser.h>
+#include <linux/tipc.h>
 #include <mtd/ubi-user.h>
 #include <net/route.h>
 
@@ -508,6 +509,7 @@ ccflags="$@"
 		$2 ~ /^XDP_/ ||
 		$2 ~ /^(HDIO|WIN|SMART)_/ ||
 		$2 ~ /^CRYPTO_/ ||
+		$2 ~ /^TIPC_/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||
 		$2 ~/^PPPIOC/ ||