Convert backup_script to backup_exec and backup

[ASDFGamer]

A setting to convert backup_script calls to backup_exec and backup calls would be very helful in mutliple situations:

1. backup calls have a better log output
2. backup calls support incremental backups

The problem with this setting is, that the files that are created/changed by backup_script have to be recognized and copied to a temp folder. This is probably relativly complicated.

[ku-we]

2. backup calls support incremental backups

The backup_script command should support incremental backups as well. As per the Debian manpage for rsnapshot:
On subsequent runs, rsnapshot checks the differences between the files created against the previous files. If the backup script generates the same output on the next run, the files will be hard linked against the previous ones, and no additional disk space will be taken up.