Adding isNodeAlive.SOCK0node

[ralmond]

Adds a method for forknodes/SOCK0nodes.

Currently, isNodeAlive for forked nodes returns NA, which is forkin' useless. This adds a check for whether or not the connection is active.

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

Seems to do what I need it to do.