Support non-ASCII TAP adapter names on Windows

Currently the TAP adapter name is fetched as an OEM string, which
is problematic if it contains non-ASCII characters and is to used
with netsh. The logfile also contains these non UTF-8 characters.

This patch fetches the name from the registry as UCS-2 and converts
it right into UTF-8 before it's used.

Signed-off-by: Heiko Hund <heiko.hund@sophos.com>
Acked-by: Josh Cepek <josh.cepek@usa.net>
Message-Id: <1381829022-15244-1-git-send-email-heiko.hund@sophos.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7913
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f2e40082349098d3c22981bf1e6d305826f1173f)