fix save_message()

fix save_message()

Prevent a crash when reopening a Mailbox.

> How exactly do these changes prevent the crash from occurring?

It's all about how mx_mbox_open() handles flags.  These four examples
are where we have a Mailbox open in 'normal' mode and try to open it
again in 'append' mode.

'Append' mode only makes sense for 'mbox' and 'compress', but the flag
affects mx_mbox_open(). It causes the function to create a duplicate
Mailbox, which is then used and discarded. The original Mailbox is now
out of sync.

The workaround, simply hides the flag for mx_mbox_open(), but
temporarily sets the internal bool append. This means that we work with
one Mailbox.