start_command
int start_command(char *, sigset_t *, int, int, ...);
pid = start_command(sh, 0, -1, pivec[1], "-c", cmdbuf, NULL);
pid = start_command(sh, &nset, image, -1, "-c", fname,
pid = start_command(value("SHELL"), &nset, fd0, fd1, "-c", cmd, NULL);