thirdshift console install on macOS, with launchd #831

From the #711 grilling, Q21. The Console (#776) is kept portable: nothing outside thirdshift console install assumes Linux or systemd, so thirdshift console --no-sign-in already runs by hand on a Mac. What's missing is keeping it running there.

Wanted: thirdshift console install on macOS. There, it writes a launchd agent that keeps thirdshift console --no-sign-in running for the logged-in user, on localhost, with no proxy. Like the Linux path, it prints what it will do rather than doing it as root.

Open questions:

• A per-user agent (~/Library/LaunchAgents), so the Console runs only while you're logged in, or a system daemon?
• Is localhost with --no-sign-in enough on a laptop, or is sign-in wanted for a laptop reachable on a LAN?
• How does it re-execute after thirdshift update? launchd's KeepAlive may already cover that.

Blocked on #828 (console install for Caddy and Apache), whose shape this follows. Needs a Mac to test on.