Expand patron identifier config options

Remove the patron_id_as_identifier yes/no configuration option and
replace it with "patron_identifier" which accepts the following:

* barcode - patron barcode (the default)
* id - patron database id

In the near future, we expect to have an additional option:

* ident - patron's ident value

This future third option will require additional work for retrieving
patrons via their ident value, etc.

Signed-off-by: Jeff Godin <jgodin@tadl.org>