Extract list reply parsing to common base class

The list and lsub replies are essentially the same, so the same class
should be able to parse both of them. We already have this implemented
in the list reply. We also want to differentiate between list and lsub
replies. We could subclass the list reply to implement the lsub reply,
but then the lsub reply would also be a list reply. What we should do is
instead to move the common code into a base class and share that with
both classes.

Create a base class for the list reply that contains the parsing common
to both the list and lsub reply.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>