LP#1736419: Located URIs vs QueryParser, round 2

LP#1736419: Located URIs vs QueryParser, round 2

The site() filter and #staff modifier are used to decide when and how to
include certain query filters, such as circ_lib or luri_org. Unfortunately,
site() is sometimes excluded (whole-tree search) and the test for staff-
iness was not specific enough, leading to incorrect queries in those cases
where information was lacking.  Now, we treat site() specially, forcing a
default of "top-of-tree", and we check for the #staff modifier directly
where necessary.

Note: this commit also addresses LP#1736992 which is about staff searches
using the limit-to-available modifier.  As a side effect of the special
site() treatment, that issue is resolved.

Signed-off-by: Mike Rylander <mrylander@gmail.com>