- Enable lxc_abstract_unix_{send,recv}_fd() to send and receive multiple fds at
  once.
- lxc_abstract_unix_{send,recv}_fd() -> lxc_abstract_unix_{send,recv}_fds()
- Send tty fds from child to parent all at once.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>