[ACD] TCP/IP stack needs to inform the application in case an address conflict has been resolved

It can happen that during the BOOTP/DHCP process an address conflict occurrs. This conflict is indicated to the application. However, when doing BOOTP/DHCP the stack in that case asks the BOOTP/DHCP server again for an address, which then might be successfully applied. In that case the stack needs to inform the application that the conflict vanished.