Start decoupling mailbox operations.

Introduce a dedicated structure for mailbox operations: struct mx_ops.
Move the open and close operations into that structure.

Assign this structure to the context in mx_open_mailbox.  This is
currently based on the "magic" for the mailbox type, but may be
refactored in the future.

Add a stub mbox_close_mailbox function.  This function does nothing,
the main purpose is to introduce a mx_ops structure for mbox, so its
usage is similar to mh/imap/pop.  We reuse the name that was made
available by the previous commmit.  Note that the actual closing of
the descriptor is done in mx.c.

To be more consistent with other mailboxes, introduce functions
mh_open_mailbox and maildir_open_mailbox, and create a dedicated
structure for mmdf.

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