Acq: Allow order responses and invoices to omit PO repetition in lineitem refs

Acq: Allow order responses and invoices to omit PO repetition in lineitem refs

Usually vendor documents have bits of EDI that look like: RFF+LI:100/123
where 100 is a PO number and 123 is a lineitem number.

Sometimes, for some documents, B&T at least will omit the '100/' part.
This is fine because we don't really need that number repeated for every
lineitem.  We do need this change so that our EDI reader code can deal
with the omission, though.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>