mariabackup: Write xtrabackup_* files as actual files even with --stream=mbstream

Right now when using the --stream=... option everything gets written to the stream only, including the xtrabackup_* files like xtrabackup_checkpoints which is required for incremental backups.

So right now streaming compression / encryption can't be combined with incremental backups without uncompressing the created streaming backup file first before taking the next incremental backup.

By having an option to write these files to both the stream and the regular target directory incremental backups would be possible without having to touch the actual stream file at all. Also the xtrabackup_* files are very small, and do not contain any sensitive information, so having these stored uncompressed/unencrypted alongside with the actual backup stream should not add any significant extra burden or security risk.

Or the xtrabackup_* files could even always be written even when using --stream mode, so that there wouldn't be a need for an extra option.