Specify the callback function to use in the server to know which
quality of protection it accepts. The quality of protection
eventually used is selected by the client though. It is currently
used by the DIGEST-MD5 mechanism. The function can be later
retrieved using gsasl_server_callback_qop_get().
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.