Prevent funny output when giving a directory name on the command line.

Before:
$ dc ..
miyu% dc ..
dc: 02 unimplemented
dc: 0 unimplemented
dc: 0 unimplemented
dc: input base must be a number between 2 and 16 (inclusive)
dc: stack empty
dc: stack empty
dc: 'h' (0150) unimplemented
dc: stack empty
dc: 'u' (0165) unimplemented
...
** get heart attack suspecting major FS corruption **

After:
$ dc ..
Cannot use directory as input!