probe-file raises an error

[Robert Morris <rtm@uun.org>]

If one calls the clisp-1995-12-08 probe-file with the name of a
directory under UNIX, it raises an error:

  > (probe-file "/tmp")
  *** - NAMESTRING: "/tmp" names a directory, not a file

CLTL1 implies probe-file should return nil in this case: "This
predicate is false if there is no file named file, and otherwise
returns a pathname that is the true name of the file." It would be
helpful to me if clisp either decided that directories are not files
(in which case probe-file should return nil), or that directories are
files.