Store the pointer to the application provided callback in the
library handle. The callback will be used, via gsasl_callback(),
by mechanisms to discover various parameters (such as username and
passwords). The callback function will be called with a
Gsasl_property value indicating the requested behaviour. For
example, for GSASL_ANONYMOUS_TOKEN, the function is expected to
invoke gsasl_property_set(CTX, GSASL_ANONYMOUS_TOKEN, "token")
where "token" is the anonymous token the application wishes the
SASL mechanism to use. See the manual for the meaning of all
parameters.