Cygwin: path_conv: Try to handle native symlinks more sanely

Cygwin: path_conv: Try to handle native symlinks more sanely

For local paths, add a check if the inner path components contain native
symlinks or junctions.  Compare the incoming path with the path returned
by NtQueryInformationFile(FileNameInformation).  If they differ, there
must be at least one native symlink or junction in the path.  If so,
treat the currently evaluated file as non-existant.  This forces
path_conv::check to backtrack inner path components until we eliminated
all native symlinks or junctions and have a normalized path.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>