makeClusterPSOCK(): Add `rscript_options`

Add argument `rscript_options` to `makeClusterPSOCK()`, cf. argument `rscript_envs`. This can be used to see R options on the cluster nodes on startup. For example,

```r
cl <- makeClusterPSOCK(1, rscript_options=list(socketOptions="no-delay"))
```

and

```r
options(socketOptions="no-delay")
cl <- makeClusterPSOCK(1, rscript_options="socketOptions")
```

Background: https://github.com/HenrikBengtsson/future/issues/437