Startup fd to avoid busywaits

Hello! I would like to propose a patch that allows the invoker to pass a "ready fd" on startup that gets written to once the setup has been completed, similar to slirp4netns's `--ready-fd` flag. Currently we have to poll the interface in a loop to wait for setup to be completed, and it would be much better if we could instead block on fd activity. Just wanted to check if such a contribution would be welcome before putting in the work of authoring it, or if there's already a better way to wait for the interface to come up. This is our current implementation: https://github.com/NixOS/nix/pull/15919/changes#diff-2a9176262efad1ef345d882... Thanks, Lisanna