bug 2582 [1/2]: don't set issuedate during renewal

bug 2582 [1/2]: don't set issuedate during renewal

Changed so that issues.issuedate is never modified
during a renewal, since that column records the original
date of the loan.  Changed the name and interpretation
of the $issuedate parameter of AddRenewal() to
$lastreneweddate, allowing (e.g.) offline circulation
to set the date of the renewal without changing the
issue date.

As a result of the original bug, issues.issuedate can be
set to NULL for loans that were renewed via the OPAC,
self checkout, or the staff interface when explicitly
renewing a loan.  Loans that were renewed by checking
the item out to the same patron will have the issue date
changed to the date of the last renewal.

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