Un-break --auth-user-pass on windows

Un-break --auth-user-pass on windows

Commit b131c7b974d9d4d3f0 introduced a check to create a meaningful
warning if we try to read a password after daemon()izing (by checking
whether stdin or stderr is connected to a tty).  For some reason this
breaks on Windows builds if run under GUI control - but since Windows
doesn't have this particular daemon() issue anyway, just #ifndef WIN32
the offending code.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1438692796-14663-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10000
(cherry picked from commit 0ffd744332f51878c4df430ab14aca3126bdfc8a)