dd: should error on reading stdin when iflag=directory option is given

GNU coreutils dd 9.4.113-c01ee

```sh
$ : | dd iflag=directory
dd: setting flags for 'standard input': Not a directory
```

uutils dd:

```sh
$ : | ./target/debug/dd iflag=directory
0+0 records in
0+0 records out
0 bytes copied, 2.37402e-04 s, 0.0 B/s
```

Test case found in GNU test suite `tests/dd/misc.sh`.

*Edit:* tested on Ubuntu 20.04, where the `iflag=directory` option is allowed.