bug 5653: use itemcallnumber in bib label layouts

bug 5653: use itemcallnumber in bib label layouts

The sample bib label layouts and the hard-coded default
format_string for new layouts used 'callnumber' when they
should have used 'itemcallnumber', preventing call numbers
from being printed on spine labels that use the system-supplied
layouts.  Besides correcting the sample data, this patch
now also enshrines 'callnumber' as an alias for 'itemcallnumber'.
Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
After applying patch it works for new and old layouts (itemcallnumber and callnumber).

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>