Review 15364

- Ensure backup scripts are run as the OSUSER (to prevent accidental running as root); 
- in scripts where LOGFILE value is changed from the 'checkpoint.log'  set by set_vars, ensure the new assignment is before check_dirs is called, otherwise errors could be written to the 'wrong' log
- in 'die()' - detect if running from terminal & also send output to stderr
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