Refresh ARP entries from incoming IP traffic

uip_arp_ipin() is an empty macro with its body under #if 0, both inherited
from the uIP import and untouched since. That leaves uip_arp_update()
reachable only from uip_arp_arpin(), so an entry is refreshed only by ARP
addressed to us and ages out after UIP_ARP_MAXAGE no matter how much traffic
passed over it.

When it goes, the next transmission to that host hits the miss path of
uip_arp_out(), which overwrites the packet in uip_buf with an ARP request and
returns. For TCP that costs a retransmission. For UDP nothing retransmits, and
handle_tx() sends the UDP connections through uip_arp_out() too, so the
datagram is lost.

The body could not be enabled as written: uip_len -= sizeof(struct uip_eth_hdr)
is a leftover from when UIP_LLH_LEN was 0. Here it is ETHER_HEADER_SIZE +
RTL_FRAME_DESC_SIZE and uip.c reads the IP header at uip_buf[UIP_LLH_LEN], so
uip_len has to keep the link layer in it. The rest of the function is sound.

rtlplayground.c

@@ -1252,7 +1252,7 @@ void handle_rx(void)
 			}
 		} else if (ETH_IN->ether_type == HTONS(0x0800)) { // IPv4
 			if (!management_vlan || management_vlan == rx_packet_vlan) {
-				uip_arp_ipin();	// Learn MAC addresses in TCP packets
+				uip_arp_ipin();
 				uip_input();
 				if (uip_len) {
 					// Add ethernet frame