Implement pipe duping

Implement pipe duping

The order of closing the ends of pipes in ex_pipeline is done because
the reading end does not receive the eof unless the writting end is
closed. That means all file descriptors pointing to the writting end
have to be closed.
If the minishell itself only closes it after execution of the second
part of a pipe, it gets deadlocked, because minishell waits for the
process end to close the pipe and the process waits for the pipe to be
closed to exit.