Used this test script:
```
<?php
require_once '../libphutil/scripts/__init_script__.php';
echo getmypid()."\n";
$future = new ExecFuture('sleep 240');
$future->resolve();
```
- Sent it SIGTERM.
- Before patch: `sleep` subprocess still alive.
- After patch: `sleep` subprocess killed.
- Sent it SIGHUP.
- Verified it dumped a trace properly.