config: test whether an item is stored in the database by name

config: test whether an item is stored in the database by name

QUERY_STRING was only used in two places, both to test whether a
variable should be stored in (or retrieved from) the database.

Since other configuration variables might be stored in the database in
the future, consolidate that test into a single function.

We also document that these configuration options should not be placed
in the config file.