Support unix exec for replacing current process

[neilyio]

It would be handy to be able to use Command::exec() with a convenient macro. I commonly use this when I need to transform the current process entirely into another process without creating a new subprocess. This can be particularly useful for creating wrappers around other programs or for use in certain types of daemon processes.

For example:

use std::os::unix::process::CommandExt;
use std::process::Command;

fn main() {
 let error = Command::new("ls")
 .arg("-l")
 .arg("/home")
 .exec(); // Replaces the current process with `ls`

 eprintln!("Failed to execute ls: {}", error);
}