Tweak the Subprocess api a bit.

From feedback from Andres changed SubProcess to just Job for
possible further changes to have the manager do things itself
without launching subprocesses.
Changed names objectpath to jobpath, exit_code to result, etc.
Removed Kill and added a force parameter to Cancel instead.
Also ran cargo fmt which tweaked indentation in a couple places.
Alos removed ExitCode, users can just call Wait again if needed.

src/bin/steamosctl.rs

@@ -233,7 +233,7 @@ async fn main() -> Result<()> {
         }
         Commands::SetWifiBackend { backend } => {
             proxy.set_wifi_backend(*backend as u32).await?;
-        },
+        }
         Commands::GetWifiBackend => {
             let backend = proxy.wifi_backend().await?;
             match WifiBackend::try_from(backend) {