@stealthcode on Linux `Process.kill('TERM', pid)` should give an exit status of 0 because it's a clean exit. Please could you update the code to use `:new_pgroup` as suggested? And also fix the windows detection as suggested.

As we're detecting windows in multiple places, let's add a `Capybara::Poltergeist.windows?` class method to keep the detection code in one place.