process: do not kill spawned processes on drop (#1814)

This updates the tokio `Command` and `Child` behavior to match that of
the stdlib: spawned processes will *not* be automatically killed when
the handle is dropped

Unlike the stdlib, any dropped (unix) processes may be reaped by tokio
behind-the-scenes after they exit and if new processes are awaited,
which mitigates the risks of piling up unreaped zombie unix processes

A `Command::kill_on_drop` method is added to allow the caller to
control whether the spawned child should be killed when the handle is
dropped. By default, this value is `false`.

The `Child::forget` method has been removed, as it is superseded by
`Command::kill_on_drop`

tokio/tests/process_issue_42.rs

@@ -33,6 +33,7 @@ fn run_test() {
                         .stdin(Stdio::null())
                         .stdout(Stdio::null())
                         .stderr(Stdio::null())
+                        .kill_on_drop(true)
                         .spawn()
                         .unwrap()
                         .boxed(),

tokio/tests/process_kill_on_drop.rs

@@ -0,0 +1,42 @@
+#![cfg(all(unix, feature = "process"))]
+#![warn(rust_2018_idioms)]
+
+use std::process::Stdio;
+use std::time::Duration;
+use tokio::io::AsyncReadExt;
+use tokio::process::Command;
+use tokio::time::delay_for;
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn kill_on_drop() {
+    let mut cmd = Command::new("sh");
+    cmd.args(&[
+        "-c",
+        "
+       # Fork another child that won't get killed
+       sh -c 'sleep 1; echo child ran' &
+       disown -a
+
+       # Await our death
+       sleep 5
+       echo hello from beyond the grave
+    ",
+    ]);
+
+    let mut child = cmd
+        .kill_on_drop(true)
+        .stdout(Stdio::piped())
+        .spawn()
+        .unwrap();
+
+    delay_for(Duration::from_secs(2)).await;
+
+    let mut out = child.stdout().take().unwrap();
+    drop(child);
+
+    let mut msg = String::new();
+    assert_ok!(out.read_to_string(&mut msg).await);
+
+    assert_eq!("child ran\n", msg);
+}