Artnet: Use uip_ipaddr_t instead of uint64 + uip_ipaddr_copy instead of ...

Code readability improvement and ipv6 requirement:
* Use uip_ipaddr_t instead of uint64
* Use "uip_ipaddr_copy(target,all_ones_addr)" to address all hosts in a subnet (aka broadcast in ipv4, multicast in ipv6)
* Use "uip_ipaddr_copy" instead of direct assignments