Curl_llist_init is never used outside of llist.c and thus it should be
static. I also removed the protos for Curl_llist_insert_prev and
Curl_llist_remove_next which are functions we removed from llist.c ages
ago.
previously had a number of flaws, perhaps most notably when an application
fired up N transfers at once as then they wouldn't pipeline at all that
nicely as anyone would think... Test case 530 was also updated to take the
improved functionality into account.