bug 5579: tweaks to bulkmarcimport.pl

bug 5579: tweaks to bulkmarcimport.pl

Fixes bug where a bib record imported by bulkmarcimport.pl
could become unindexable by ensuring that ModBiblioMarc()
is always called by bulkmarcimport.pl to finalize saving the
bib record (as it was initially created by AddBiblio with the
defer_marc_save option).

Also introduces a utility routine, C4::Biblio::_strip_item_fields.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>