@param config The neo4j client configuration to update.
@param callback The callback to be invoked whenever a host verification
fails.
@param userdata User data that will be supplied to the callback.
@return 0 on success, or -1 on error (errno will be set).
Set the unverified host callback.
@param config The neo4j client configuration to update. @param callback The callback to be invoked whenever a host verification fails. @param userdata User data that will be supplied to the callback. @return 0 on success, or -1 on error (errno will be set).