compact and more efficient code.
* grub-core/kern/list.c (grub_list_push): Moved from here ...
* include/grub/list.h (grub_list_push): ... to here. Set prev.
(grub_list_remove): Moved from here ...
* include/grub/list.h (grub_list_remove): ... here. Use and set prev.
(grub_prio_list_insert): Set prev.
* include/grub/list.h (grub_list): Add prev. All users updated.