- Rename devif_input() ipv4_input()

- Copy net/devif/devif_input.c to ipv6_input.c.  Remove all IPv4-specific logic.
- Rename net/devif/devif_input.c to ipv4_input.c.  Remove all IPv6-specific logic
- Split IPv4 header structure out as net_ipv4hdr_s from net_iphdr_s

drivers/net/enc28j60.c

@@ -1384,7 +1384,7 @@ static void enc_rxdispatch(FAR struct enc_driver_s *priv)
     {
       nllvdbg("IP packet received (%02x)\n", BUF->type);
       arp_ipin(&priv->dev);
-      devif_input(&priv->dev);
+      ipv4_input(&priv->dev);
 
       /* If the above function invocation resulted in data that should be
        * sent out on the network, the field  d_len will set to a value > 0.