to_upper_fd
n = to_upper_fd(n);
fd2 = to_upper_fd(fd); /* closes fd, returns higher equiv */
ttyfd = to_upper_fd(ttyfd); /* Move to a high fd */
outx->fd = to_upper_fd(dup(out2->fd));
int to_upper_fd(int);
fd = to_upper_fd(fd);