tap.c: fix adapter renaming

tap.c: fix adapter renaming

Turns out that renaming adapter by setting registry key doesn't
really work - while new adapter name is shown in control panel
etc, when one tries to change adapter properties (like set DNS)
with netsh call - it fails:

Fri Mar 13 09:05:36 2020 us=569311 Setting IPv4 dns servers
on 'OpenVPN Wintun' (if_index = 14) using service
Fri Mar 13 09:05:37 2020 us=118028 TUN: adding IPv4 dns failed
using service: Funktio ei kelpaa.   [status=1 if_name=OpenVPN Wintun]

This renames adapter with netsh command, like:

    netsh interface set interface
          name="Local Area Connection 2" newname="OpenVPN Wintun"

Above functionality is used by tapctl.exe and openvpnsica.dll
(during installation).

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Simon Rozman <simon@rozman.si>
Message-Id: <20200703192029.306-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20207.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>