Bug 15485: (QA followup) Simplify logic and use *ResultsDisplay value during upgrade

Bug 15485: (QA followup) Simplify logic and use *ResultsDisplay value during upgrade

This patch makes the new sysprefs work as the previously defined. Instead of falling back
to what *ResultsDisplay is set, it now has its own 'default' (that still defaults to the
*Results*.xslt).

The default values are set to 'default' as the rest of XSLT-related sysprefs, and the upgrade
picks whatever is set in OPACXSLTResultsDisplay and XSLTResultsDisplay so current behaviour
is preserved.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
New XSLTs for Lists work as advertised

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>