Implement splitted backup support on multiple ISO images

As backups can now be included inside the ISO image, it can be usefull to split them onto several ISO images. (eg: burn to CD or DVD)
I introduced the ISO_MAX_SIZE option (default unset, so feature disabled) which will be the maximum size of the backup in any ISO image.
The first ISO image name isn't changed. Additional ISO images are called with the same name but with a number as suffix.
All ISO images are pushed to the OUPTUT_URL as before.
A file called backup.splitted is generated and is used to know if a backup is splitted or not. This file also contains several informations about generated images.

About restore, it's a bit more complex. I use a fifo pipe to be able to restore backup from multiple DVD/ISO without interrupting the restore process. About the wait for next DVDs, a loop is used to detect automatically when a new DVD is inserted (by volid). When integrity check is enabled, backup checksum are checked when each new DVD is inserted as it isn't possible to do that at the beginning of the recover.

As this feature introduced a lot of new code, it has been checked in a lot of configuration and is, according to all my tests, regression free.