net: implement addrcmp for AF_UNSPEC

net: implement addrcmp for AF_UNSPEC

When mtr is compiled with IPv6 enabled, the GTK frontend sets the address family to AF_UNSPEC when changing destinations. However, addrcomp does not support this AF type, which causes all comparisons to not match. For the GTK frontend, this leads to a cascading failure when setting the destination to a host that does not resolve:

1. Upon setting ctl->af = AF_UNSPEC, net_max() will report that all hops have data, as no hops can match the previous remote address.
2. The GTK frontend tries to render a row for every hop, including its IP / hostname.
3. The guard in the GTK frontend to avoid looking up unknown addresses also fails, causing mtr to flood DNS packets and effectively hang.

Fixes https://github.com/traviscross/mtr/issues/533