sort: omit 'restrict' in doubtful cases

sort: omit 'restrict' in doubtful cases

* src/sort.c (lock_node, unlock_node, queue_destroy, queue_init):
(queue_pop):
Omit 'restrict'; it shouldn't help here, as these functions have just
one pointer parameter and don't access static storage.
(queue_insert, check_insert, update_parent): Omit 'restrict', as
the pointer types differ, and are not char * or unsigned char *,
and therefore can't alias.
(write_unique): Omit 'restrict', as the pointer types are all
read-only.
(merge_loop, sortlines): Omit 'restrict', as any performance
advantages are extremely unlikely and it's not worth cluttering
the code for that.
(struct thread_args): Omit 'restrict': this seems to be incorrect.
It's unlikely for 'restrict' to be correct inside a typedef.