1.0.28.69: filesystem tests and small Windows improvements

1.0.28.69: filesystem tests and small Windows improvements

* FILE-AUTHOR returns NIL instead of signalling an error on Windows

* Missing DIRECTORY canonicalization tests.

* Check one-letter devices for being alpha-chars when unparsing
  them on Windows.

* NATIVE-NAMESTRING now has similar tailing-slash handling
  on Windows as elsewhere -- adjust the test.

* Windows namestrings canonicalize / to \ -- make the random
  namestring tests take that into account.

  ...filesys.pure.lisp passes on Windows.