cmd/tailscale: status --json should include stable ID

[bradfitz]

User notes about tailscale status --json:

The only field that I can't find in the json output, though, is a deviceId that I could pass directly to the REST API.

I see that I can use the REST API to gather the list of all devices, and match up the tailscale IP address or other details to find the deviceId, but it would be nice to avoid the extra work!

Front conversations

[rosszurowski]

I believe the REST API uses the legacy IDs right now, though it should probably support both. We should ensure that whatever IDs we expose to end users work on the API too, before making that change.

[DentonGentry]

It looks like we did this, somewhere along the way:

    "nodekey:0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef": {
      "ID": "nabc012CNTRL",
      "PublicKey": "nodekey:0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
      "HostName": "server1",

[bradfitz]

Done in e72ed3f