Support IPv6 link-local addreses

[daurnimator]

They currently don't work as inet_pton doesn't support link local addresses.

Looks like the only POSIX way to parse them is getaddrinfo. We could use the AI_NUMERICHOST hint flag?

[daurnimator]

I could easily enough come up with a patch to move src/lib/socket.c's so_open off of inet_pton and just use dns.c

However dns.c uses inet_pton too. and changing dns.c to support the scope field will be a quite a large change; especially if we want to support it everywhere. I've made an issue over there about it: wahern/dns#6