[#1990] Improve support for links


Problem description
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The previous code had a isLink method, but is_link was not available as part of getAttributes



Changes description
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Added getStatus as the base method for getting file/folder propereties.

With getStatus I have fixed a stupid design where getAttribute was returning a raw stat structure... this was a  quick hack required for SFTP.

I have refactored isLink isFolder isFile to be shortcuts for getAttribute method.

I added tests for the new code.

After this, I will start working on adding link support for Windows.


How to try and test the changes
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reviewers @alibotean 

check that code is ok.

Thanks!