Allow creation of directories under the volume root in Win32

We ran into an issue where cloning a repository to a folder
directly underneath the root of a volume (e.g. 'd:\libgit2')
would fail with an access denied error. This was traced down
to a call to make a directory that is the root (e.g. 'd:') could
return an error indicated access denied instead of an error
indicating the path already exists. This change now handles
the access denied error on Win32 and checks for the existence
of the folder.