bugfix: do not try to set items.cn_sort twice

bugfix: do not try to set items.cn_sort twice

The way this API is used by additem.pl, the $item hasref
already contains a 'items.cn_sort' key whose value is
undef.  Setting $item->{'cn_sort'} instead of $item->{'items.cn_sort'}
ends putting with items.cn_sort and cn_sort in the UPDATE
items statement, making the final value of cn_sort in the DB
dependent on the order produced by Perl's keys function.

Also added a comment explaining a subtle (perhaps too subtle)
point in the code.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>