Only process stamps if no --id is passed

Instead of requiring a number of mirrors, which is error-prone if a lot
of --ids are passed.
Do not re-enable mirrors if stamps are not going to be processed

Leave the log message as an indicator.
Make the maximum subset age configurable by mirror type

The default is still 13 hours.
Really only access {$type}{arch}{$arch} if it exists
Mirrors with all architectures provide any architecture

(yes, really)