Add additional environment variable post backup BACKUP_STATUS for partial

SYSTEM INFORMATION
OS type and version Debian
Webmin version Latest
Virtualmin version Latest

Good morning @staff,

I would like to propose a new BACKUP_STATUS environment value of “2” when a backup is partial. We use central software to keep track of many Virtualmin backups and the post backup command only allows 0 or 1 for fail or success.

Please review my pull request: