Provide list convenience methods

All methods common to both list and lsub replies were previously moved
to a common base class. This did also make it so that all common access
methods had to be called on the base class and the object had to be type
casted. This is inconvenient to do all the time and it would be nice if
we could call a method directly on the object even though it is
implemented in the base class.

Add convenience methods to the list reply that calls the base class
methods, making it unnecessary to do the type cast when working with
list replies.

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