Since 0.2.4.11-alpha (in 0196647970) we've tried to randomize
the start time to up to some time in the past. But unfortunately we
allowed the start time to be in the future as well, which isn't
really legit.
The new behavior lets the start time be be up to
MAX(cert_lifetime-2days, 0) in the past, but never in the future.
Fixes bug 21420; bugfix on 0.2.4.11-alpha.
