more items work; handle unlinked subfields (DB rev => 048)

more items work; handle unlinked subfields (DB rev => 048)

As a consequence, the items table now remains authoritative
for all item information: any subfields not explicitly mapped
in the framework are still stored.

In addition, the additional unlinked subfields are now retained
during a call to ModItem.

Two additional points of note:

* an upgrade script from 2.2 and earlier 3.0 instllations is needed
  to populate items.marc_subfields_xml; I am working on this.
* A behavour of bulkmarcimport.pl has now changed; any item subfields
  not explicitly mapped to a Koha items field were dropped; they are
  now retained and are stored in items.more_subfields_xml.  This can
  affect data migrations.

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