[lwip-users] Set IP address as none

[Giuseppe Modugno]

I have ip4_addr_t and I want to set it to a "null"/invalid value. Is it 
better to use IPADDR_NONE or IPADDR_ANY?

At first, I thought IPADDR_NONE was the best choice, but I found that 
dns.c uses IPADDR_ANY to flag a DNS server as invalid.

Moreover, there are ip4_addr_isany() and ip4_addr_set_any() macros, 
while there aren't ip4_addr_isnone() and ip4_addr_set_none().