cbtype - Callback type. One of the types listed in Callback Types.
callback - Pointer to custom callback function. The prototype for
this function must match the associated callback declaration macro.
Sets a custom callback handler for a previously initialized session
object. Callbacks are triggered by the receipt of special packets at
the Transport layer. To disable a callback, set it to NULL.
RETURN VALUE
Pointer to previous callback handler. Returns NULL if no
prior callback handler was set.