Reset logging to default if no explicit value was set

This fixes #56. Note that the approach is slightly less conservative than what I initially proposed: There is no check whether the default value has changed since the otr conversation was started. I believe this is more in line with what the user expects.

There is also a trivial commit that changes `test_until_fail.sh` to use bash, as `let` (and `(( ))` expansion) is not POSIX-compliant.