Parse LSUB replies similar to LIST replies

The LSUB reply is essentially the same as the LIST reply. The actual
parsing was therefore earlier extracted into a common base class to make
it simple to support both with the same parser.

Teach libmx to parse LSUB replies by adding a new subclass that
implements the LSUB reply.

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