Always use STDIN_FILENO for input and STDOUT_FILENO

Always use STDIN_FILENO for input and STDOUT_FILENO
for output, to avoid confusion with closed input and output fds.
(input_fd, output_fd): Remove; all uses changed to STDIN_FILENO
and STDOUT_FILENO.
(open_fd): New function.
(main): Use it, instead of open, to ensure that file descriptors
don't get confused.

(skip): Don't fstat the input file; the result is no longer used.