[NET]: Add ARPHRD_NONE and use it in tun driver.

drivers/net/tun.c

@@ -129,8 +129,8 @@ static void tun_net_init(struct net_device *dev)
 		dev->addr_len = 0;
 		dev->mtu = 1500;
 
-		/* Type PPP seems most suitable */
-		dev->type = ARPHRD_PPP; 
+		/* Zero header length */
+		dev->type = ARPHRD_NONE; 
 		dev->flags = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
 		dev->tx_queue_len = 10;
 		break;

include/linux/if_arp.h

@@ -86,6 +86,7 @@
 #define ARPHRD_IEEE80211_PRISM 802	/* IEEE 802.11 + Prism2 header  */
 
 #define ARPHRD_VOID	  0xFFFF	/* Void type, nothing is known */
+#define ARPHRD_NONE	  0xFFFE	/* zero header length */
 
 /* ARP protocol opcodes. */
 #define	ARPOP_REQUEST	1		/* ARP request			*/