bib and item editing bugfixes

bib and item editing bugfixes

- bugfix: update items.cn_sort upon edit of item
- bugfix: make failure of a value_builder plugin non-fatal
- bugfix: remove references to non-existent plugins
          from optional MARC21 frameworks
             bookseller.pl
             marc21_callnumber.pl
             marc21_classcodes.pl
             marc21_locationqualifier.pl
- bugfix: corrected trying to enter 'u*' fields twice in the 'CF'
          format in the optional MARC21 frameworks; resulted in
          most of the framework not being loaded

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