On Windows, don't try to set "stdin" nonblocking, as it does not work
See also https://bugs.php.net/bug.php?id=34972.
Note that you can't ^C during a prompt (or at any other time) in Windows currently, see T13549.
Test Plan: On Windows, hit a prompt in "arc land", then answered it successfully.
Differential Revision: https://secure.phabricator.com/D21358