Bug 21172: Remove warning from addbiblio.pl

Bug 21172: Remove warning from addbiblio.pl

Argument "01e" isn't numeric in numeric ne (!=) at
/home/vagrant/kohaclone/cataloguing/addbiblio.pl line 507.

A tagfield can be 01e and so raise a warning is compared with == or !=

This patch also fixes few inconsistencies:
- tagfield for items.itemnumber must be retrieved with GetMarcFromKohaField
(to support other marcflavours)
- do not use $_
- loop only once on $usedTagsLib

Test plan:
For devs only, read the code and confirm the changes make sense.

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>