[R] mclapply hanging occasionally on macos

[Ivo Welch]

just a plain macos desktop, latest sequoia.  no clusters or anything
complex.  when I ^c out of it, a `traceback()` on macos tells me that

```
[highest number]: selectChildren(ac[!fin], -1)
[-1]: mclapply(...
```

I don't know what selectChildren is waiting for (internal?  macos?),
but it certainly is patient.  I wonder whether others have encounted
similar problems on occasion.  it's not a huge deal, but it would be
nice to stop...unstop.