command: Add process_group method to Command struct

Rust 1.64 stabilised the `process_group` method of the
`std::os::unix::process::CommandExt` trait. This allows the
caller to set the process group that a command is spawned in.

Expose that functionality from Tokio's `Command` structure.

This fixes tokio-rs#4312.

tokio/src/process/mod.rs

@@ -690,6 +690,28 @@ impl Command {
         self
     }
 
+
+    /// Sets the process group ID (PGID) of the child process. Equivalent to a
+    /// setpgid call in the child process, but may be more efficient.
+    ///
+    /// Process groups determine which processes receive signals.
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    ///
+    /// Command::new("ls")
+    ///		.process_group(0)
+    ///		.spawn()?
+    ///		.wait()?;
+    /// ```
+    #[cfg(unix)]
+    #[cfg_attr(docsrs, doc(cfg(unix)))]
+    #[cfg(tokio_unstable)]
+    pub fn process_group(&mut self, pgroup: i32) -> &mut Command {
+        self.std.process_group(pgroup);
+        self
+    }
+
     /// Executes the command as a child process, returning a handle to it.
     ///
     /// By default, stdin, stdout and stderr are inherited from the parent.