Bug 2505: adding warnings to opac-detail.pl

Bug 2505: adding warnings to opac-detail.pl

Before starting to edit opac/opac-detail.pl, I added 'use warnings' and
eliminated all of the warnings that I could get it to produce. I also
extracted two lines that prepared an amazon ISBN into a function. They
were causing a few warnings and might as well be extracted. They should
probably be moved to a module somewhere since these two lines are copied
all over our code.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>