On Windows, don't try to set "stdin" nonblocking, as it does not work

Summary:
See https://discourse.phabricator-community.org/t/arc-land-fail-unable-to-set-stdin-nonblocking/4006/.

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