ssh/sftp: detect that we are not in a physical root and print a meaninful error/warning

[efiop]

We might be able to detect that by comparing ssh ls -la vs sftp ls #2755 (comment) , but need to confirm that it is reliable enough. Also, there might be another better way to do that.

[efiop]

This would help users that try to use an ssh remote without knowing that it is configured to have an sftp root that is different from a physical root on the machine. See #2535 and #2755 (comment) for more info.

[efiop]

Closing as stale.