IP_ADDR_ANY macro

Can be used as a fixed/const ip_addr_t for the IP wildcard. Defined to IP4_ADDR_ANY when IPv4 is enabled. Defined to IP6_ADDR_ANY in IPv6 only systems. Use this if you can handle IPv4 _AND_ IPv6 addresses. Use IP4_ADDR_ANY or IP6_ADDR_ANY when the IP type matters.