Add config option for storing a diagnostic device.

For some setups with an autochanger you might want to query the
individual drives for tape alerts. As you cannot open the tape
device twice you need to access the drive via a SCSI generic
device. We now have a per device diagnostic device config variable
which you can expand using a %D in the tape alert cmdline. Normally
you should query the autochanger for tape alerts and that should also
report any tape drive errors but some devices implement this poorly and
as such it doesn't work. This option allows you to work around that and
actually ask the drive for any tape alerts.

Fixes #200: Storage Daemon Tape Alert Useless / Defective