1. Create a new osrfListExtract function, which removes an item

1. Create a new osrfListExtract function, which removes an item
from an osrfList without destroying it, and returns a pointer to
the item thus removed.

2. Create a new jsonObjectExtractIndex, which removes a
specified entry in a JSON_ARRAY, and returns a pointer
to it, without destroying it.

3. In osrf_json.h: Corrected an inaccurate comment about
jsonObjectRemoveIndex().  Contrary to the original comment, this
function does not shift other objects down to fill the gap.

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1689 9efc2488-bf62-4759-914b-345cdb29e865