Specify the callback function to use in the server for checking if
a GSSAPI user is authorized for username (by, e.g., calling
krb5_userok()). The function should return GSASL_OK if the user
should be permitted access, or an error code such as
GSASL_AUTHENTICATION_ERROR on failure. The function can be later
retrieved using gsasl_server_callback_gssapi_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.