Bug 26473: Get items for editing using only itemnumber

Bug 26473: Get items for editing using only itemnumber

This patch modifies additem.pl so that it can accept an itemnumber as
a single parameter.

To test, apply the patch edit items on a bibliographic record which has
one or more items.

- In the location bar, modify the URL so that it uses only the
  itemnumber parameter, e.g.

  /cgi-bin/koha/cataloguing/additem.pl?itemnumber=12345

  The page should load the correct record's items.

- If you add the "op" parameter it should load the item for editing in
  the form:

  /cgi-bin/koha/cataloguing/additem.pl?itemnumber=12345&op=edititem

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>