[WIN32] fixed: support for long paths broke file access on UNC shares

Gotham users reported some problems with network share access using the MediaPortal pvr addon with as result broken Live TV. After some testing I saw that the recent addition of @wsoltys to support local paths with more than MAX_PATH characters broke the access to \\server\share\ paths. 

The correct extended length name for UNC paths is \\?\UNC\server\share and not \\?\\server\share
(see also http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx)

This pull requests corrects the changes from commit faaf4ef9d70f8540062be47bd7a5cd96f09dba1d
for UNC shares.