caches-queues-lists: implement dump_seq_queue

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
caches-queues-lists: get rid of readp/writep in dump_queue

The exact value of readp/writep isn't really of interest,
and it could be confusing that the seq_queue doesn't
have then.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
caches-queues-lists: make queue dump more informative

Rather than giving the name of the queue, describe what the
queue does!

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>