kernels using the fallback have an inherent close-on-exec race
condition and as such support for them is only best-effort anyway.
however, ignoring potential new flags is still very bad behavior.
instead, fail with EINVAL.
if (!flag) return pipe(fd);
int ret = __syscall(SYS_pipe2, fd, flag);
if (ret != -ENOSYS) return __syscall_ret(ret);
+ if (flag & ~(O_CLOEXEC|O_NONBLOCK)) return __syscall_ret(-EINVAL);
ret = pipe(fd);
if (ret) return ret;
if (flag & O_CLOEXEC) {