Rework DNS and routes in network extension

Rework DNS and routes in network extension

The DNS resolver prior had useless comments, awful nesting, converted
bytes into strings and back into bytes, and generally made no sense.
That's been rewritten now.

But more fundumentally, this commit made the DNS resolver actually
accomplish its objective, by passing AI_ALL to it. It turns out, though,
that the Go library isn't actually using GAI in the way we need for
parsing IP addresses, so we actually need to do another round, this time
with hints flag as zero, so that we get the DNS64 address.

Additionally, since we're now binding sockets to interfaces, we can
entirely remove the excludedRoutes logic.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>