SshScpFIle implementations of canRead/canWrite/canExecute are incomplete

[hierynomus]

The current implementation for the SshScpFile methods canRead, canWrite and canExecute are incomplete and incorrect.

If the user is not the owner of the file, the current implementations return incorrect values.

It's better to:

1. check whether the user == owner, and in that case look at the first section of the lsResults,
2. check whether the user is in the group that owns the file, then check the second section
3. else check the third section