[PATCH] blackbox: add logreadonlybyuser to restrict permission to log (issue5065)

[timeless]

first, I'd rather start by making the log not world readable.

> +            logreadonlybyuser = self.hasconfig('blackbox', 'logreadonlybyuser')

second, we typically use configbool(..., ..., False).
I'm not sure what hasconfig is/does, but I suspect it's the wrong thing.
e.g --config blackbox.logreadonlybyuser=False

third, I'm not a fan of the name.

and finally, I want to take some time to think about this before we
take it. (other feedback is welcome)