Adding support for hard-linked directories and files

[t-w]

As mentioned in lclevy/ADFlib#23, here are some small modifications for:

• adfOpenFile() - adding support for opening files using their hard links
• adfChangeDir() - adding support for entering hard-linked directories.

(both with some simple functional tests)

[lclevy]

see https://github.com/lclevy/ADFlib/blob/master/arccsh.adf for a CShell to create links

[lclevy]

Thanks for your work