Clean up structure of io/console and avoid stty on Windows.

[headius]

This should fix #3989.

I have restructured io/console to be more modular and now force Windows to always use the stubbed version...never the stty version.

Other platforms should work as they did before, but the code is a bit easier to follow now.

[headius]

Request review from @kares, @enebo. This is almost structurally identical to the old one, but the old logic was hard to follow and had unusual guards (like the ttymode defined check) that I did not carry over.

I believe the new logic is cleaned and more appropriate:

1. If Windows, use stubbed always.
2. If Linux or BSD, attempt to use native; if that fails, continue with (3)
3. Attempt to use stty. If that fails, fall back to stubbed.

[kares]

👍 such a cleanup would be good to have on 9K (pretty much all in one file)
although I am not sure how deep its modified compared to the stock version for MRI (due stdlib imports).

[headius]

In MRI, io/console is written entirely in C. Part of this reorg is to get files like linux_console.rb out of the io load namespace. I realize I have added a new namespace with JRuby-specific files, but I think it is isolated pretty well.

I will get this into 9k as well since they're both from the same sources.