napt: Fixes and improvements

 1. Fix enable/disable to properly allocate and deallocate tables.
    Current algorithm is just broken.
 2. Introduce eviction policy when table gets full: oldest connection
    is evicted, instead of new ones getting silently dropped. this
    results in much better behavior with small tables than before.
    When TCP connection is dropped, RSTs are sent both ways to inform
    parties instead of dropping silently. thiw requires additional 8
    bytes per entry but is, again, a big improvement for clients in
    terms of usability.
 3. FIxed handling of timestamp wraparound (every ~50 days of uptime).
 3. Added ip_portmap_get() to retrieve current port mapping settings.
 4. Added ip_napt_get_stats() for some insight into the state of NAT.

src/core/ipv4/dhcp.c

@@ -364,14 +364,15 @@ dhcp_check(struct netif *netif)
 static void
 dhcp_handle_offer(struct netif *netif, struct dhcp_msg *msg_in)
 {
+  u8_t n;
   struct dhcp *dhcp = netif_dhcp_data(netif);
 
   LWIP_DEBUGF(DHCP_DEBUG | LWIP_DBG_TRACE, ("dhcp_handle_offer(netif=%p) %c%c%"U16_F"\n",
               (void *)netif, netif->name[0], netif->name[1], (u16_t)netif->num));
 
   /* Vendor Specific Information */
 #if ESP_DHCP && !ESP_DHCP_DISABLE_VENDOR_CLASS_IDENTIFIER
-  for (u8_t n = 0; (n < DHCP_OPTION_VSI_MAX) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_VSI + n); n++) {
+  for (n = 0; (n < DHCP_OPTION_VSI_MAX) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_VSI + n); n++) {
     dhcp_option_vsi[n] = lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_VSI + n));
   }
 #endif /* ESP_DHCP && !ESP_DHCP_DISABLE_VENDOR_CLASS_IDENTIFIER */