Expose host-resolver as a stand-alone daemon

[jandubois]

BTW if we can just fix libslirp to use the sane resolver, that will be helpful for non-Lima QEMU users and we can avoid CGO.

https://gitlab.freedesktop.org/slirp/libslirp/-/blob/v4.6.1/src/slirp.c#L244

Originally posted by @AkihiroSuda in #281 (comment)

[jandubois]

I think that embedding our DNS server inside libslirp won't work because libslirp is written in C, so we can't use the Go DNS library.

But I think we could expose our server as a standalone daemon that would serve e.g. on 127.0.0.53:53 and then we could put that address as the only address into /etc/resolv.conf on the host (the same way systemd-resolved works).

That way libslirp would automatically inherit it (I hope, on macOS it doesn't read /etc/resolv.conf, but calls libresolv, so needs more research). But also all other tools, like dig, host, or nslookup would suddenly start using the native macOS resolver.

For Lima I would still run the code inside the hostagent, so that we don't need an extra daemon, and more importantly, so that we don't need to mess with /etc/resolv.conf. But it would be nice if we could make this available system-wide with just a launch daemon for other use cases.