DNS resolver support

[sunfishcode]

getaddrinfo, freeaddrinfo, and gai_strerror. I found a simple Rust library for doing simple DNS queries, so I'm going to look into implementing the c-scape parts to hook it up.

Another option would be to use something like trust_dns_resolver, however my guess is that people who want that are using trust_dns_resolver directly, and that what std wants is just something small and simple, similar to what libcs typically provide through getaddrinfo. I may be wrong here, and we'll see as mustang matures, but I think this will work for now.

[sunfishcode]

I'll be working on this soon, and I'll work on basic getsockopt/setsockopt functionality needed to support it.