Restructure code in BackupJob class and subclasses

Pass in the backup plan and let the jobs read info directly from there. Makes code shorter and more flexible to add or remove settings which are read by backup jobs.
Clean up printing of messages to log file from backup jobs
Add bup archive verification as an optional feature.

Run before backing up to make sure it does not interfere with a possible par2 generation.
Disable "Generate recovery info" option if par2 is not installed

Looks a bit better to inform user that an option is actually not available.