Some of the GET iterator initializers modify their get_data_t arg, so
this allows these initializers to be passed to get_next.

In particular this allows init_note_iterator_all to be passed to
get_next, for a fix to handle_get_notes.  Note that
init_note_iterator_all itself does not modify its get_data_t arg, but
init_note_iterator_all is passed to select_resource_iterator which
also accepts initializers that do modify their get_data_t args.

As a result drop the const from any initializer passed to get_next.