Rename completion helpers

We have io_uring_get_sqe() on the submission side, yet the completion
side is named _completion. Rename as follows:

io_uring_get_completion()	io_uring_peek_cqe()
iO_uring_wait_completion()	io_uring_wait_cqe()

This better tells the user what the _get variant does by calling it
_peek instead, and we move to using _cqe() as the postfix instead
of _completion.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

test/cq-full.c

@@ -62,7 +62,7 @@ int main(int argc, char *argv[])
 
 	i = 0;
 	do {
-		ret = io_uring_get_completion(&ring, &cqe);
+		ret = io_uring_peek_cqe(&ring, &cqe);
 		if (ret < 0) {
 			printf("wait completion %d\n", ret);
 			goto err;