add option to enable/disable background job verbose logging

[agowa]

When a mirror transfer is started and then put into the background via CTRL+Z, it keeps writing its verbose (-v) output to the terminal (unexpected, but technically not an issue so far), but there is no way to stop it (the issue).

Proposed change: entering "debug off" should also disable the verbose mode of backgrounded jobs (or add a "verbose on/off" command)