Implement support for sending the check command

The check command requests a checkpoint of the currently selected
mailbox. Add support for sending the check command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Implement support for sending the close command

The close command removes all messages marked with the \Deleted flag
from the currently selected mailbox and returns to the authenticated
state. Add support for sending the close command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Implement support for sending the expunge command

The expunge command removes all messages marked with the \Deleted flag
from the currently selected mailbox. Add support for sending the expunge
command.

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