Restrict file paths for FilesystemClient syscalls

[Robin Krahl]

All functions in FilesystemClient should make sure that the client does not escape its dat namespace.

trussed/src/store/filestore.rs

Lines 43 to 49 in 9b9662d

 pub fn actual_path(&self, client_path: &PathBuf) -> PathBuf { 
     let mut path = PathBuf::new(); 
     path.push(&self.client_id); 
     path.push(&PathBuf::from("dat")); 
     path.push(client_path); 
     path 
 }