*sigh*

Note to self and others: Don't use "archive and install, preserve users & network settings" when downgrading from Panther dev preview to Jaguar. It makes it so that network doesn't get set up at all, and users get dropped to console when they try to log in. And apparently I am too clueless to set up a working network connection with ifconfig and route. (It may have worked if I had not accidentally specified en0 at first, but then I couldn't undo that mistake.)