Change DNS resolution to be async

[kyle-github]

There are some other possibilities than the old POSIX gethostbyname() call. We need async resolution and cancellation.

• glibc has getaddrinfo_a / gai_cancel
• Windows has GetAddrInfoExW / GetAddrInfoExCancel
• OpenBSD has getaddrinfo_async / asr_abort
• macOS has CFHostStartInfoResolution / CFHostCancelInfoResolution
• FreeBSD has dnsres_getaddrinfo, but no cancellation support

(cribbed from a Zig announcement)

It is unfortunate that the BSDs are not unified in this.