1.0.28.60: partial UNC pathname support for Windows

1.0.28.60: partial UNC pathname support for Windows

  UNC hosts are represented using the devíce components of pathnames,
  as are drives. This is sleightly lossy since it prevents accessing
  network hosts named with a single letter -- single-letter devices
  are taken to mean drives.

  However, since storing the host in the pathname host component
  would lead to confusion between logical hosts and UNC hosts,
  this seems preferable right now, so that

   (make-pathname :host "foo" ...)

  remains unambiguous. DIRECTORY does not work yet with UNC pathnames
  since it insists on walking the path from root -- which Windows
  doesn't seem to allow for UNC paths, not even local ones.