Rework DHCP PCB handling: Old code registered one UDP PCB per netif where DHCP was active and there was a special case in udp_input() for this. New implementation uses one PCB for all netifs and removes special case in udp_input().

The old approach called udp_bind() on each of the PCBs, which puts them into udp_pcbs list. The PCBs were iterated on all non-DHCP udp_inputs() with no effect.
My cleanup removes the special handling in udp.c, and uses only one DHCP UDP PCB to catch all DHCP messages from all netifs. The dhcp_recv function then checks whether ip_current_input_netif() has DHCP enabled - if not, the message is ignored. The PCB is only created/registered when one or more PCBs have DHCP enabled.

src/include/lwip/dhcp.h

@@ -59,10 +59,10 @@ struct dhcp
 {
   /** transaction identifier of last sent request */
   u32_t xid;
-  /** our connection to the DHCP server */
-  struct udp_pcb *pcb;
   /** incoming msg */
   struct dhcp_msg *msg_in;
+  /** track PCB allocation state */
+  u8_t pcb_allocated;
   /** current DHCP state machine state */
   u8_t state;
   /** retries of current request */