Tailscale CLI has poor UX with expiring keys

Via https://twitter.com/QuinnyPig/status/1536835238327746561

If you `tailscale up` in a network, eventually the key will expire. Being a CLI rather than a persistent GUI, `tailscale` has no way of proactively telling you that expiry is nigh. However, even if you know that expiry is coming, the UX isn't great:

- [ ] `tailscale status` doesn't warn you that expiry is imminent, or that expiry has happened. We could add a health report for that.
- [ ] `tailscale up` when you're in the "your key expires soon" window doesn't reauth, it just silently exits after concluding there's nothing to do. You can `--force-reauth` to actually go through the auth again, but we don't tell you that anywhere.

There's limited things we can achieve with a non-persistent CLI, but here's a couple we can do:

- [ ] Add a health report about key expiry, so that `tailscale status` and our debugging machinery surfaces it as an issue with the system
- [ ] Add a nudge to `tailscale up` where, if you're still authed but the key is up for expiry soon, we print "you're still authed, but your key's up for renewal soon. Use `--force-reauth` to do so now". Or, since there should be no downside to reauthing eagerly, maybe we make `tailscale up` automatically `--force-reauth` when we're in the window for reauth warnings?

cc @bradfitz @maisem for thoughts