Move mbox close-append logic inside mbox_close_mailbox().

Move mbox close-append logic inside mbox_close_mailbox().

The mx_fastclose_mailbox() calls mx_ops->close(), which invokes
mbox_close_mailbox().

Also, close the ctx->fp inside mbox_close_mailbox().  This way, the
(to be added) compress logic can call the mx_ops->close() instead of
"knowing" to close the fp before recompressing.

mx_fastclose_mailbox() will safe_fclose() the fp again, but I'm
leaving it there just in case I missed a usage of the fp in some other
part of the code.

Thanks to Damien Riegel for the original patch.