Correctly memoize our knowledge of the existence of stty

Three states are required for memoizing whether stty exists: yes, no & unknown. The prior implementation does not distinguish between the no & unknown states. On Wiindows, where stty is not likely to exist, this results in #stty_exists? rechecking every time for existence of stty.
Merge pull request #460 from cablegram/windows-fix-stty

Correctly memoize our knowledge of the existence of stty