backup original config file

The backup-my.cnf generated during a backup only contains options needed by ibbackup.

I suggest a new feature that would make Xtrabackup capable of saving the original my.cnf - under a different name - with the backup(original-my.cnf).

It's really not a big deal to copy it with a separate command but it would be a handy feature during semi-automated replication building(plus having another config backup stored with the data couldn't do any harm)