Use `O_DUPFD` instead of `dup`

[bgamari]

In #282 we introduced some rather subtle dup logic to ensure that the fork pipe fds don't shadow stdin/stdout/stderr. However, it turns out that POSIX provides a much simpler means of doing this: fcntl(fd, O_DUPFD) (manpage). We should rather use this.

[expipiplus1]

or O_DUPFD_CLOEXEC for the CLOEXEC pipes, to avoid racing another process forking before setting CLOEXEC in a separate call

[bgamari]

Yes, fair point.