Use setsid() instead of setpgid()

This makes it possible to run commands that allocates TTYs and still
capture their output and reap the process on completion.

> setpgid — set process group ID for job control
> setsid — create session and set process group ID

With this in place, the `waitpid` call detects that the process has
exited even if the process allocated a TTY, and is able to unblock the
runner and continue.

Further, the output of the command is also shown in the controlling
terminal.

src/process.rs

@@ -114,7 +114,7 @@ mod imp {
             }
 
             command
-                .before_exec(|| setpgid(Pid::from_raw(0), Pid::from_raw(0)).map_err(from_nix_error))
+                .before_exec(|| setsid().map_err(from_nix_error).map(|_|()))
                 .spawn()
                 .and_then(|p| {
                     Ok(Process {