* group urlio data into an `URLState` structure
* allocate `URLState` in `url_init()` and store it into the `QEmacsState`
* pass the `URLState` pointer to all `url_xxx` APIs
* add `tail_cb` for redisplay requests
* remove global variables
* simplify `qe_init()` call sequence
* add `QE_LIST_INIT` to initialize a linux doubly linked list