TPAC: Add preferred library to search result ordering

TPAC: Add preferred library to search result ordering

Now that unapi.bre/unapi.holdings_xml has learned how to order copies by
ranked library, teach TPAC how to tell unapi what the preferred library
is. We do this by parsing the preferred search library separately from
the scoped search library, as follows (1st match wins):

1) A new GET param, "plib", specifying the number of the OU
2) User's library preference setting;
3) User's home OU;
4) Physical location in which the search is being issued

Signed-off-by: Dan Scott <dan@coffeecode.net>