UDS: align struct sockaddr_un with NetBSD

Well, make a start, anyway. Our copy was missing a legacy field from
the structure, that could very well cause applications to fail trying
to set, clear, or check it. As a consequence, SUN_LEN now yields the
same result as on NetBSD.

Change-Id: I80f6aff7769be402b3bd3959f64d314509ed138c

sys/sys/un.h

@@ -16,6 +16,7 @@ typedef __sa_family_t	sa_family_t;
  * Definitions for UNIX IPC domain.
  */
 struct	sockaddr_un {
+	uint8_t		sun_len;
 	sa_family_t	sun_family;
 	char		sun_path[UNIX_PATH_MAX];
 };