Allow reading stdin on Windows instead of forcibly opening console

Allow reading stdin on Windows instead of forcibly opening console

This was filed against Ľubomír Carik's github project for openconnect-gui;
https://github.com/openconnect/openconnect-gui/issues/101

It isn't perfect, as the ANSI code page on Windows can be different
from the OEM code page used for the console, so fgetws() is likely
to do the wrong thing — which is why we force-opened the console and
used ReadConsoleW() in the first place. But perfect is the enemy of
good in this case, as reading from something other than stdin is
*definitely* wrong. We still use ReadConsoleW() when stdin does happen
to be the console, so that part shouldn't regress.

I hate Windows...

Signed-off-by: David Woodhouse <dwmw2@infradead.org>