[ipv4] Use a zero address to indicate "no gateway", rather than INADDR_NONE

ipv4.c uses a gateway address of INADDR_NONE to represent "no
gateway".  It initialises the gateway address to INADDR_NONE before
calling fetch_ipv4_setting() to retrieve the configured gateway
address (if any).

However, as of commit 612f4e7 "[settings] Avoid returning
uninitialised data on error in fetch_xxx_setting()",
fetch_ipv4_setting() will zero the IP address if the setting does not
exist, rather than leaving it unaltered.

Fix by using a zero IP address to indicate "no gateway", so that a
non-existent gateway address setting will be treated as such.

src/net/ipv4.c

  * @v netdev		Network device
  * @v address		IPv4 address
  * @v netmask		Subnet mask
- * @v gateway		Gateway address (or @c INADDR_NONE for no gateway)
+ * @v gateway		Gateway address (if any)
  * @ret miniroute	Routing table entry, or NULL
  */
 static struct ipv4_miniroute * __malloc
 
 	DBG ( "IPv4 add %s", inet_ntoa ( address ) );
 	DBG ( "/%s ", inet_ntoa ( netmask ) );
-	if ( gateway.s_addr != INADDR_NONE )
+	if ( gateway.s_addr )
 		DBG ( "gw %s ", inet_ntoa ( gateway ) );
 	DBG ( "via %s\n", netdev->name );
 
 	/* Add to end of list if we have a gateway, otherwise
 	 * to start of list.
 	 */
-	if ( gateway.s_addr != INADDR_NONE ) {
+	if ( gateway.s_addr ) {
 		list_add_tail ( &miniroute->list, &ipv4_miniroutes );
 	} else {
 		list_add ( &miniroute->list, &ipv4_miniroutes );
 
 	DBG ( "IPv4 del %s", inet_ntoa ( miniroute->address ) );
 	DBG ( "/%s ", inet_ntoa ( miniroute->netmask ) );
-	if ( miniroute->gateway.s_addr != INADDR_NONE )
+	if ( miniroute->gateway.s_addr )
 		DBG ( "gw %s ", inet_ntoa ( miniroute->gateway ) );
 	DBG ( "via %s\n", miniroute->netdev->name );
 
 	list_for_each_entry ( miniroute, &ipv4_miniroutes, list ) {
 		local = ( ( ( dest->s_addr ^ miniroute->address.s_addr )
 			    & miniroute->netmask.s_addr ) == 0 );
-		has_gw = ( miniroute->gateway.s_addr != INADDR_NONE );
+		has_gw = ( miniroute->gateway.s_addr );
 		if ( local || has_gw ) {
 			if ( ! local )
 				*dest = miniroute->gateway;
 	struct settings *settings;
 	struct in_addr address = { 0 };
 	struct in_addr netmask = { 0 };
-	struct in_addr gateway = { INADDR_NONE };
+	struct in_addr gateway = { 0 };
 
 	/* Delete all existing routes */
 	list_for_each_entry_safe ( miniroute, tmp, &ipv4_miniroutes, list )
 		/* Override with subnet mask, if present */
 		fetch_ipv4_setting ( settings, &netmask_setting, &netmask );
 		/* Get default gateway, if present */
-		gateway.s_addr = INADDR_NONE;
 		fetch_ipv4_setting ( settings, &gateway_setting, &gateway );
 		/* Configure route */
 		miniroute = add_ipv4_miniroute ( netdev, address,

src/usr/route.c

 		printf ( "%s: %s/", miniroute->netdev->name,
 			 inet_ntoa ( miniroute->address ) );
 		printf ( "%s", inet_ntoa ( miniroute->netmask ) );
-		if ( miniroute->gateway.s_addr != INADDR_NONE )
+		if ( miniroute->gateway.s_addr )
 			printf ( " gw %s", inet_ntoa ( miniroute->gateway ) );
 		if ( ! ( miniroute->netdev->state & NETDEV_OPEN ) )
 			printf ( " (inaccessible)" );