Perform one step of SASL authentication in server. This reads data
from client (specified with input and input_len), processes it
(potentially invoking callbacks to the application), and writes
data to client (into variables output and output_len).
The contents of the output buffer is unspecified if this functions
returns anything other than GSASL_NEEDS_MORE.
Returns GSASL_OK if authenticated terminated
successfully, GSASL_NEEDS_MORE if more data is needed, or error