enumerated value of Gsasl_property type, indicating the
type of data in data.
Retrieve the data stored in the session handle for given property
prop, possibly invoking the application callback to get the value.
The pointer is to live data, and must not be deallocated or
modified in any way.
This function will invoke the application callback, using
gsasl_callback(), when a property value is not known.
If no value is known, and no callback is specified or if the
callback fail to return data, and if any obsolete callback
functions has been set by the application, this function will try
to call these obsolete callbacks, and store the returned data as
the corresponding property. This behaviour of this function will
be removed when the obsolete callback interfaces are removed.
Return data for property, or NULL if no value known.