Seed /var/lib/athena-update-status at install time

[jdreed]

Between when a machine is reinstalled and it first takes an update (which can be as long as 6 hours), Nagios will complain about "Can't parse output" for the "UPDATE" service.

The installer should seed /var/lib/athena-update-status with the following values:
$time|$time|ok|Fresh install

where $time is the current time in seconds since the epoch (i.e. date +%s)

[jdreed]

r24942

I did this in the postinst of auto-update, which seems more correct. This means that in theory there will be a short period of Nagios whining while athinfod is running, but before auto-update has been configured, but don't think we care.