Specify the callback function to use in the client to inform the
server of the largest buffer the client is able to receive when
using the DIGEST-MD5 "auth-int" or "auth-conf" Quality of
Protection (qop). If this directive is missing, the default value
65536 will be assumed. The function can be later retrieved using
gsasl_client_callback_maxbuf_get().
DEPRECATED
This function is part of the old callback interface.
The new interface uses gsasl_callback_set() to set the application
callback, and uses gsasl_callback() or gsasl_property_get() to
invoke the callback for certain properties.