process: make Child::kill async (#2823)

* This changes the `Child::kill` to be an async method which awaits the
  child after sending a kill signal. This avoids leaving zombie
  processes on Unix platforms if the caller forgets to await the child
  after the kill completes
* A `start_kill` method was also added on `Child` which only sends the
  kill signal to the child process. This allows for kill signals to be
  sent even outside of async contexts.

tokio/tests/process_issue_2174.rs

@@ -39,8 +39,7 @@ async fn issue_2174() {
     time::delay_for(Duration::from_secs(1)).await;
 
     // Kill the child process.
-    child.kill().unwrap();
-    let _ = child.wait().await;
+    child.kill().await.unwrap();
 
     assert_err!(handle.await);
 }