Add ChildExt::kill_process_group

This function wraps POSIX `killpg(pid, SIGKILL)`, and on Linux
additionally may be implemented by `pidfd_send_signal`.

library/std/src/os/unix/process.rs

@@ -452,6 +452,36 @@ pub trait ChildExt: Sealed {
     /// ```
     #[unstable(feature = "unix_send_signal", issue = "141975")]
     fn send_process_group_signal(&self, signal: i32) -> io::Result<()>;
+
+    /// Forces the child process's process group to exit.
+    ///
+    /// This is analogous to [`Child::kill`] but applies to every process in
+    /// the child process's process group.
+    ///
+    /// Use [`CommandExt::process_group`] to assign a child process to an
+    /// existing process group, or to make it the leader of a new process group.
+    /// By default spawned processes are in the parent's process group.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(unix_kill_process_group)]
+    ///
+    /// use std::{os::unix::process::{ChildExt, CommandExt}, process::{Command, Stdio}};
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut child = Command::new("cat")
+    ///         .stdin(Stdio::piped())
+    ///         .process_group(0)
+    ///         .spawn()?;
+    ///     child.kill_process_group()?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`Child::kill`]: process::Child::kill
+    #[unstable(feature = "unix_kill_process_group", issue = "156537")]
+    fn kill_process_group(&mut self) -> io::Result<()>;
 }
 
 #[unstable(feature = "unix_send_signal", issue = "141975")]
@@ -463,6 +493,10 @@ fn send_signal(&self, signal: i32) -> io::Result<()> {
     fn send_process_group_signal(&self, signal: i32) -> io::Result<()> {
         self.handle.send_process_group_signal(signal)
     }
+
+    fn kill_process_group(&mut self) -> io::Result<()> {
+        self.handle.send_process_group_signal(libc::SIGKILL)
+    }
 }
 
 #[stable(feature = "process_extensions", since = "1.2.0")]