- remove error.SharingViolation from all error sets since it has the
  same meaning as FileBusy
- add error.FileBusy to CreateFileAtomicError and ReadLinkError
- update dirReadLinkWindows to use NtCreateFile and NtFsControlFile and
  integrate with cancelation properly.
- move windows CTL_CODE constants to the proper namespace
- delete os.windows.ReadLink