- The code required to prepare an fd to act as a login tty is shared among
  pty_on_host_callback() and fork_pty(). This implements login_pty(), a
  minimalistic login_tty() clone, to avoid code redundancy.
- Give pty_in_container() a slightly extended comment.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>