More customization markers, and general cleanup

First, add more CUSTOMIZATION markers wherever a value likely needs
per-system configuration.

Second, clean up and clarify the code in a few places.  This includes
standardizing some variable names.  In particular, $visid and
$barcode were both used interchangeably, and this commit standardizes
on $visid for anything which isn't an actual local item barcode (i.e.
it is a fake or external system "barcode" value).

Signed-off-by: Dan Wells <dbw2@calvin.edu>