Bug 4421: Add alternate holdings display and prefs

Bug 4421: Add alternate holdings display and prefs

This patch adds the ability to specify a field with alternate holdings
information for display when a biblio has no items associated with it.

Two sysprefs are added:
* AlternateHoldingsField specifies what field/subfields contain the alternate
holdings information. When blank, the alternate holdings information is not
displayed. The default is blank, as this is a new feature.
* AlternateHoldingsSeparator specifies the string to be used to separate
multiple subfields in the alternate holdings display. The default is ' '.

Example use case:
A library which does not have a 1-1 relationship between uncontrolled 852 fields
from a legacy system and actual physical items on the shelf wishes to display
holdings information from the 852, but does not want to create item records
which are almost certain to be inaccurate. By enabling the alternate holdings
feature (AlternateHoldingsField = '852abcdhi' and AlternateHoldingsSeparator =
' -- '), the library is able to gradually add item records as they locate the
physical items, without losing the holdings information presently stored in the
uncontrolled 852 fields.

To test:
1) Set AlternateHoldingsField to '852abcdhi'
2) Set AlternateHoldingsSeparator to ' -- '
3) Change the hidden value of subfields 'a', 'b', 'c', 'd', 'h', and/or 'i' of
   field 852 to 0 so that they display
4) Create a record which has data in the 852, but no item record
5) Look at holdings tab, where the data you entered should be displayed

Proof-of-concept initially developed for the American Numismatic Society.

Signed-off-by: Jared Camins-Esakov <jcamins@bywatersolutions.com>
Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>