Use biblioitems.itemtype as a default for items.itype

Use biblioitems.itemtype as a default for items.itype

This correct the html select default in additem.pl, and enforces use of
biblioitems.itemtype as the default in Items::AddItem.  The code in
Items is to catch items added through MARC record staging.

Thanks to Joe Atzberger for suggestion.

[RM note: this patch means that an item's item-level item type,
 once that item is added or updated via additem.pl,
 now cannot be blank or NULL unless the biblio-level item type
 is blank.]

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>