Load wintun.dll from the application directory only

Load wintun.dll from the application directory only

Do not attempt to load it from the System32 directory.

Different versions of `wintun.dll` or `wintun.sys` float around in system
directories. In my case, a `C:\Windows\System32\wintun.sys` had been left
behind for some reason, and was being loaded at startup, taking precedence
over the `wintun.dll` bundled with OpenConnect. Unfortunately, different
versions are not compatible, at least not entirely, while OpenConnect is
being tested with the bundled `wintun.dll` only.

To avoid this DLL hell, we shall load exclusively the bundled version of
`wintun.dll` from the application directory, and disregard any `wintun.dll`
or `wintun.sys` installed in system directories by other software.

Signed-off-by: Dimitri Papadopoulos <3350651-DimitriPapadopoulos@users.noreply.gitlab.com>