OPAC: Make advanced search -> numeric search -> bib cn hone in on right target

OPAC: Make advanced search -> numeric search -> bib cn hone in on right target

The bib call number search under the advanced search page's numeric
search pane was doing the wrong thing.  By thinking the search field
desired was called 'bib_cn' where it's actually called 'bibcn', it
generated searches that just fell back to the broader search class
'identifier'.  This would often find your matches, but would also find
too many records that *shouldn't* have been matches.

Discovered with help from Erica Rohlfs and Mike Rylander.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>