Support non-ASCII characters in Windows tmp path

Get the temporary path from the system as UCS-2 and convert it to
UTF-8 and return that for internal use.

Fix trac#278.

Signed-off-by: Heiko Hund <heiko.hund@sophos.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1384882574-28242-1-git-send-email-heiko.hund@sophos.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8002
Signed-off-by: Gert Doering <gert@greenie.muc.de>