Special-case the last alert for alert-key-compromised case

Hard-code a special nId=max int alert, to be broadcast if the
alert key is ever compromised. It applies to all versions, never
expires, cancels all previous alerts, and has a fixed message:

URGENT: Alert key compromised, upgrade required

Variations are not allowed (ignored), so an attacker with
the private key cannot broadcast empty-message nId=max alerts.