Fix ref. to nonexistent $visid, clarify terms

The XML for NCIP message RenewItemResponse was being constructed
with a nonexistent variable $visid, resulting in warnings and no
content in the ItemIdentifierValue element.

When processing a RenewItem message in renew_item(), we are
referencing the item by its UniqueItemId, so in addition to fixing
the nonexistent variable issue, this commit adjusts the variable
names for clarity.

Log output is also changed.

Signed-off-by: Jeff Godin <jgodin@tadl.org>