[wish] simplify tape serial usage

[retokromer]

• add an optional flag which allows to use the bar code label as the tape serial
• add the regex that checks the consistency of the bar code label for this not-standard use (this secondary regex would be used instead of the primary one when the flag is set)
• delete the use of an additional global variable

I guess this would be a better fix for https://github.com/amiaopensource/ltopers/issues/167.