Bug 10859: (follow-up) clarify logic on multiple loans on same bib

Bug 10859: (follow-up) clarify logic on multiple loans on same bib

This patch clarifies the logic for determining if a given item to be
checked out would be the second (or third, etc.) loan on the same bib.

As a conseqence, if the item is already on loan to the patron, the
circ staffer won't see the multiple-loans-on-a-bib warning, just
the confirmation to renew the loan or the warning that no more
renewals are lest.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>