Fix handling of LWIP_HOOK_VLAN_SET(). Previous implementation supplied uninitialized arguments to the macro (struct eth_hdr).

Change macro signature to be universal: netif, pbuf, src, dst, eth_type - whatever the user needs to decide about VLAN header.
Return value <0 means "no VLAN header", 0 <= return_value <= 0xFFFF -> value is prio_vid of header.
Clean up ethernet_output function to be more readable.

UPGRADING

@@ -57,6 +57,8 @@ with newer versions.
      * added hook LWIP_HOOK_MEMP_AVAILABLE() to get informed when a memp pool was empty and an item
        is now available
 
+  * Signature of LWIP_HOOK_VLAN_SET macro was changed
+
   * LWIP_DECLARE_MEMORY_ALIGNED() may be used to declare aligned memory buffers (mem/memp)
     or to move buffers to dedicated memory using compiler attributes