Bug 11603: If a letter code is given, filenames should contain it

Bug 11603: If a letter code is given, filenames should contain it

If you choose to generate print notices for a specific letter code, the
generated files should be distinct.

The use case is: you want to process print notice for letter codes:
overdue1, overdue2 and overdue3.

The cronjobs will be:

perl misc/cronjobs/gather_print_notices.pl
/tmp --letter_code=overdue1 --csv --ods --html --delimiter=";"

perl misc/cronjobs/gather_print_notices.pl
/tmp --letter_code=overdue2 --csv --ods --html --delimiter=";"

perl misc/cronjobs/gather_print_notices.pl
/tmp --letter_code=overdue3 --csv --ods --html --delimiter=";"

without this patch, the 2 first files will be erased.

Signed-off-by: Frederic Demians <f.demians@tamil.fr>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>