Add forknode method for isNodeAlive

(Please use for Q&A)

**Wish or feature request**

`isNodeAlive()` currently returns `NA` no matter the state of the cluster, which is forkin' useless.

Suggest adding a method for `SOCK0node` which checks the connection status.

```
ccc <- makeForkCluster(2)
> isNodeAlive(ccc)
[1] NA NA
> isNodeAlive.SOCK0node <- function (x, ...) isConnectionValid(x$con)
> isNodeAlive(ccc)
[1] TRUE TRUE
> stopCluster(ccc)
> isNodeAlive(ccc)
[1] FALSE FALSE
>
```

See futureverse/parallelly#136

Start by locating the SOCK0node implementation and the existing isNodeAlive() and isConnectionValid() methods. Check how makeForkCluster() and stopCluster() represent connection state, then verify that isNodeAlive() reports TRUE for live nodes and FALSE after stopping the cluster, as shown in the issue example.

Written by the indexing model from the issue text.