Implement support for sending LSUB commands

Since libmx now knows about LSUB replies and can parse them we should be
able to send LSUB commands. Implement support for sending the LSUB
command from mxsh similar to how LIST commands are sent. Catch the reply
once it is received and show the name of the mailbox.

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