Changes in some settings and refactoring

This is backwards incompatible, but probably nobody is using these
* `linetrace+` is now `linetrace_plus`
* `trace` is now `stack_trace_on_error`
`Byebug` instead of `byebug` when showing version
Fix small bug in test suite

We want to reset CommandProcessor state between tests, not only the
interface. For example, @last_line and @last_file were not being reset.